Learn to Multiply Decimals (Decimal Multiplication) - [15] - By Math and Science
Transcript
| 00:00 | Hello . Welcome back . The title of this lesson | |
| 00:02 | is called multiplying decimals . This is part one . | |
| 00:05 | I'm excited to teach you this because in the last | |
| 00:07 | lesson we've learned what multiplying decimals really means in terms | |
| 00:11 | of pictures and models and just thinking through what it | |
| 00:13 | is here . We're actually going to practice skills so | |
| 00:16 | we're not using any pictures . We're going to write | |
| 00:18 | the problem down and solve it now . I want | |
| 00:20 | to tell you a couple things before we do our | |
| 00:22 | first problem . The first thing is when we multiply | |
| 00:24 | these numbers with decimals , basically we're going to completely | |
| 00:28 | ignore the decimal point until the final answer . So | |
| 00:31 | the way that we have already learned how to multiply | |
| 00:33 | numbers is really the same as what we're going to | |
| 00:36 | do here . We're going to pretend the decimals aren't | |
| 00:38 | even there were going to get the answer and then | |
| 00:41 | at the very end I'll tell you where to put | |
| 00:43 | the decimal point . So really it's not going to | |
| 00:45 | be any different . We're very much different from what | |
| 00:47 | you already know . The second thing is when we | |
| 00:50 | were adding and subtracting decimals . I told you you | |
| 00:52 | must line up the decimal points . I told you | |
| 00:55 | 30,000 times when you add or subtract to line the | |
| 00:58 | decimal points up . When we multiply or when we | |
| 01:01 | divide we do not line up decimal points . The | |
| 01:04 | lining up the of the decimals only is for adding | |
| 01:06 | and subtracting here . When we multiply , we do | |
| 01:09 | not line up decimal points . We we don't do | |
| 01:12 | that . So let me show you how we're going | |
| 01:13 | to do our first problem . This problem actually is | |
| 01:16 | the same problem . We did when we in the | |
| 01:19 | first Lesson , when we did the by the models | |
| 01:21 | here we did a problem that looked like this 04 | |
| 01:25 | And we were multiplying by 0.5 , Now in this | |
| 01:30 | case the decimal points happen to line up but I'm | |
| 01:33 | not lining them up on purpose , I'm just doing | |
| 01:35 | it because the columns are lining up and the decimals | |
| 01:37 | happen to fall here . We're gonna multiply now if | |
| 01:40 | you remember from the last lesson we said that the | |
| 01:44 | basically just ignore the decimal point until the final answer | |
| 01:47 | . So what I want you to do is just | |
| 01:49 | pretend the decimal isn't even there . Like just pretend | |
| 01:52 | at 04 and 05 And we multiply , what do | |
| 01:55 | we do ? We start in the right column five | |
| 01:57 | times four is what 20 ? Right ? So we | |
| 01:59 | put a zero here and we carry a two over | |
| 02:01 | here then zero times 50 And then we add the | |
| 02:06 | two which means we get it too . So we | |
| 02:08 | put it to here . All right now we're done | |
| 02:10 | with that . So then we have to multiply by | |
| 02:12 | zero so we could drop a zero down here and | |
| 02:15 | then we multiply zero times four we get zero and | |
| 02:18 | we multiply zero times zero and we get zero . | |
| 02:20 | We don't add this too because that was from a | |
| 02:22 | previous step . Now , whatever we get from this | |
| 02:25 | , we have to add up the zeros , give | |
| 02:27 | me a zero , the 20 give me two . | |
| 02:30 | And the zero just goes right here . So you | |
| 02:32 | see what we've done is completely ignore the decimal point | |
| 02:35 | . The decimal point is we just pretend it's not | |
| 02:37 | there right ? And we get the numbers and the | |
| 02:39 | answer and then now I have to show you how | |
| 02:41 | to put the decimal in the final answer . What | |
| 02:44 | you do is you look back to your problem and | |
| 02:47 | you ask yourself , how many digits do I have | |
| 02:49 | after the decimal here I have a decimal 0.0.0.1 digit | |
| 02:52 | after the decimal . And here I have a decimal | |
| 02:54 | point with another digit after the decimal . So for | |
| 02:57 | the whole problem I only have two digits after decimal | |
| 03:00 | points , one from here and one from here . | |
| 03:02 | So in my final answer I have to have two | |
| 03:04 | digits after the decimal .1 from here and one from | |
| 03:08 | here . So I have to go do I put | |
| 03:10 | it here ? No if I put it here I'm | |
| 03:12 | only gonna have one digit after the decimal . So | |
| 03:14 | I have to actually put it right here . 0.20 | |
| 03:19 | 0.20 is the final answer when you take 0.4 and | |
| 03:23 | cut it in half which is what you're doing by | |
| 03:25 | multiplying by .5 . The answer we got from the | |
| 03:28 | pictures is 0.20 . We did that by shading and | |
| 03:32 | chopping and counting and all that . That was just | |
| 03:34 | for you to understand what's happening and we now have | |
| 03:37 | put that aside and now we're calculating and the way | |
| 03:40 | we calculate is we do not . The decimals just | |
| 03:43 | happened to line up here but we do not have | |
| 03:45 | to line them up like this , we line the | |
| 03:47 | columns up multiply as usual , pretend the decimals aren't | |
| 03:50 | even there and then we have one digit after the | |
| 03:53 | decimal . Here's another digit after the decimal for a | |
| 03:55 | total of two . So the decimal must go here | |
| 03:57 | . So we have two digits after the decimal in | |
| 04:00 | our final answer . And notice that really if you | |
| 04:02 | just cover up all everything four times 54 times five | |
| 04:05 | is 20 . Here's your 20 we just have to | |
| 04:08 | put the decimal in the right spot . So really | |
| 04:10 | when you're multiplying by decimals it's like the decimals they | |
| 04:13 | don't really affect the numbers of the answer . But | |
| 04:16 | the decimals in the problem , tell you where the | |
| 04:18 | decimal point goes in your answer ? That's what it | |
| 04:21 | does now . Instead of multiplying the decimal times , | |
| 04:25 | a decimal , let's change things a little bit to | |
| 04:27 | something , you know that I think it's fairly easy | |
| 04:29 | for us to understand . Let's say that I have | |
| 04:33 | the number 0.5 and we're going to multiply it by | |
| 04:38 | four . Now notice the four has an invisible decimal | |
| 04:41 | at the end four point . Oh right , but | |
| 04:44 | I'm not lining up the decimals because I only line | |
| 04:46 | them up when I'm adding . When I'm multiplying , | |
| 04:49 | I don't need to line them up because the answer | |
| 04:51 | , the decimal point does not just come down . | |
| 04:53 | If it were to come down it would land here | |
| 04:56 | . I have to count decimals when we multiply . | |
| 04:58 | So here I'm not lining decimals up , I'm just | |
| 05:01 | putting things there . Let's multiply . But let's also | |
| 05:03 | think , what do we think the answer is going | |
| 05:04 | to be if I have four sandwiches and cut them | |
| 05:07 | in half by multiplying . I should only have two | |
| 05:09 | sandwiches left . So we think the answer should be | |
| 05:11 | too right . And so let's check it out . | |
| 05:15 | Four times five is what 20 ? Put a zero | |
| 05:18 | carry the to ignore the decimal . Four times zero | |
| 05:21 | is what ? Zero ? We add the two and | |
| 05:24 | we get it too . So here's our 20 . | |
| 05:26 | Now we look at our problem . How many digits | |
| 05:28 | after the decimal here ? We have one . How | |
| 05:30 | many digits after the decimal here ? There is no | |
| 05:33 | decimal point and there's no numbers after . So we | |
| 05:35 | don't have any uh digits after decimal there at all | |
| 05:38 | . So in the problem we only have one digit | |
| 05:41 | after the decimal . So in our answer we must | |
| 05:44 | have one digit after the decimal . So the answer | |
| 05:46 | to this would be two point oh so if you | |
| 05:48 | have four sandwiches and you cut that in half by | |
| 05:51 | multiplying by 40.5 you should have two sandwiches left , | |
| 05:54 | that's exactly what this is , exactly . Two sandwiches | |
| 05:57 | left . So the rest of these problems are going | |
| 05:59 | to behave the same way . We're going to write | |
| 06:02 | the problem down and we're going to solve it essentially | |
| 06:06 | by ignoring the decimal 1.2 times six . This is | |
| 06:12 | like taking a little bit more than a sandwich , | |
| 06:14 | 1.2 sandwiches and multiplying six times so making six copies | |
| 06:18 | and adding them all together , multiplying by six we | |
| 06:21 | ignore the decimal . We pretend the decimal isn't even | |
| 06:24 | there six times two is what , 12 ? So | |
| 06:27 | we put a to carry a one , Six times | |
| 06:29 | one is 6 , we add one more and we | |
| 06:31 | get seven . So those are our digits . That's | |
| 06:34 | it . Now . In our problem we only have | |
| 06:36 | one digit after the decimal . This one we have | |
| 06:38 | no decimal we have no numbers after we only have | |
| 06:41 | one digit after the decimal . So it must go | |
| 06:43 | here . One digit after the decimal . For the | |
| 06:46 | answer to be 7.2 . All right , 7.2 . | |
| 06:52 | Let's take a look at problem at the next problem | |
| 06:56 | . Let's say we have 0.61 and we're gonna multiply | |
| 07:01 | by three again . I want to show you that | |
| 07:03 | there's an invisible decimal point here . Were did not | |
| 07:05 | line the decimals up . We did not line the | |
| 07:08 | decimals up here and we did not line the decimals | |
| 07:10 | up here . We do not do that for multiplying | |
| 07:12 | or dividing . We only do it for adding and | |
| 07:14 | subtracting . I know it's a little weird in the | |
| 07:16 | beginning but you're gonna have to get used to it | |
| 07:19 | . We push everything over just like regular multiplication problem | |
| 07:22 | . What is three times one ? Well that's three | |
| 07:26 | . What is three times six ? That's 18 . | |
| 07:28 | So we put in eight carry the one three times | |
| 07:31 | zero is 31 more . I'm sorry three times zero | |
| 07:34 | is zero anytime anything times zero is zero plus one | |
| 07:38 | is one . And now for the decimal point we | |
| 07:41 | have two digits after the decimal nothing here . So | |
| 07:44 | in their final answer we must have two digits after | |
| 07:47 | the decimal . The answer is 1.83 Alright next problem | |
| 07:53 | . Let's say we have 0.58 and we're gonna multiply | |
| 07:57 | that times 12 . Again there's an invisible decimal after | |
| 08:01 | the 12 . We have not lined it up with | |
| 08:03 | this , we just push the numbers over into the | |
| 08:05 | columns And we multiply starting with the 22 times eight | |
| 08:09 | is what 16 ? So we put a six carry | |
| 08:12 | a one five times two is what ? 10 ? | |
| 08:14 | One more is 11 . So we have a one | |
| 08:17 | and we have to carry 12 times 001 more is | |
| 08:21 | one . So we're done with that . Now we | |
| 08:23 | have to multiply by the one so we dropped 01 | |
| 08:26 | times eight is 81 times five is five . We | |
| 08:30 | don't have to add these . That's from the previous | |
| 08:32 | step , one times 0 is zero . And then | |
| 08:35 | we add okay what do we get for the answer | |
| 08:38 | ? Six plus zero is 68 plus one is 95 | |
| 08:42 | plus one is six . And now we do have | |
| 08:44 | a zero plus zero but we don't really put leading | |
| 08:47 | zeros in numbers . So the answer is 696 But | |
| 08:50 | where does the decimal go ? Our problem has two | |
| 08:53 | digits after the decimal and then no digits after decimal | |
| 08:56 | . For a total of two . There must then | |
| 08:58 | be two digits after the decimal in our final answer | |
| 09:01 | and then the answer is 6.96 . So if I | |
| 09:04 | take .58 which is a little bit more than half | |
| 09:07 | of a sandwich and I multiply that times 12 and | |
| 09:11 | add it all together . Then I'm going to have | |
| 09:13 | almost seven sandwiches basically . That's what that really means | |
| 09:17 | . A little bit less than 7696 is a little | |
| 09:19 | bit less than seven . Yeah . All right . | |
| 09:23 | Let's take a look at problem number four . Problem | |
| 09:28 | number four , let's say we have 1 to 1 | |
| 09:33 | and we're going to multiply that times three . So | |
| 09:36 | this means I have a little bit more than one | |
| 09:38 | sandwich . 1.21 sandwiches and I'm going to triple it | |
| 09:41 | . What am I going to have in the end | |
| 09:44 | ? Three times one is what ? Three ? Three | |
| 09:46 | times two is 63 times one is again three And | |
| 09:50 | then for the decimal , I have two digits after | |
| 09:52 | the decimal here and none here . So my answer | |
| 09:54 | must have two digits after the decimal , which means | |
| 09:56 | I go right here . And the answer is 3.63 | |
| 10:00 | . Yeah . All right . Here's the halfway mark | |
| 10:03 | . Let's say we have 32.7 and we're gonna multiply | |
| 10:08 | that times two . So basically we're going to double | |
| 10:10 | it . What do we have ? Seven times two | |
| 10:14 | ? Is what ? 14 carry the 12 times two | |
| 10:17 | is four , add one more which is five , | |
| 10:20 | Two times three is six . That's all the multiplying | |
| 10:23 | now for our or decimal we have one digit after | |
| 10:26 | the decimal and nothing here . So told that we | |
| 10:29 | have to have one digit after the decimal point for | |
| 10:32 | 65.465.4 . All right , let's take a look at | |
| 10:41 | 3.7 and will multiply that times 1.5 . Now notice | |
| 10:46 | in this case the decimals are lined up but that's | |
| 10:49 | not because I'm lining them up on purpose . It's | |
| 10:51 | because the columns are lined up in such a way | |
| 10:54 | where I have to have the columns lined up and | |
| 10:56 | so the decimals happen to be here . I push | |
| 10:58 | everything to the right . The columns are lined up | |
| 11:00 | . The decimals are not lined up in these problems | |
| 11:02 | . It's just a coincidence in this problem . So | |
| 11:05 | let's start by multiplying by 55 times seven is what | |
| 11:09 | 35 ? That means I put a five and I | |
| 11:11 | carry a three , Three times five is 15 . | |
| 11:15 | Then we add 16 , 17 , 18 . So | |
| 11:18 | I have to put the 18 right here . Now | |
| 11:21 | we multiply by the one first we have to drop | |
| 11:23 | a zero and one time 77 and one times three | |
| 11:26 | is three . I don't add this because that was | |
| 11:29 | from before . Shit . Finally we have to add | |
| 11:34 | these numbers five plus zero gives us 58 Let's go | |
| 11:38 | up by 79 10 , 11 , 12 , 13 | |
| 11:41 | , 14 15 . We land on 15 . So | |
| 11:44 | we have to put a five here and we carry | |
| 11:46 | on one to the next column 345 goes here . | |
| 11:50 | So we have 555 now . Where does the decimal | |
| 11:53 | go ? In ? Our problem we have one decimal | |
| 11:55 | digit after the decimal here and one digit after the | |
| 11:58 | decimal here . For a total of two . So | |
| 12:00 | then we have to have then the decimal here . | |
| 12:02 | We have to have two digits after the decimal one | |
| 12:04 | from here and one from here . Now it's important | |
| 12:07 | for you to kind of realize that that what we're | |
| 12:10 | doing is counting digits after the decimal and counting digits | |
| 12:13 | after the decimal . You might think that you just | |
| 12:15 | drop that digit that decimal down . But if you | |
| 12:17 | just drop it down , you would probably think it | |
| 12:19 | lands here . Um so you can't just guess . | |
| 12:22 | You need to count how many digits after the decimal | |
| 12:24 | and then put it in the proper location . All | |
| 12:28 | right . So let's take a look at our final | |
| 12:32 | four problems . Let's take 6.24 and will multiply it | |
| 12:40 | times 2.8 . Notice that these decimals are not lined | |
| 12:45 | up . The way I have it is the columns | |
| 12:47 | are lined up with the numbers but the decimal points | |
| 12:49 | are not . Uh and that's the way we do | |
| 12:51 | multiplication . We don't line the decimals up . Start | |
| 12:54 | by ignoring the decimal , multiply by 88 times four | |
| 12:58 | is 32 . So we put up to here carry | |
| 13:01 | a three , eight times two is 16 . Going | |
| 13:04 | up 17 , 18 , 19 . We put a | |
| 13:06 | nine carry a 18 times six is 48 . 1 | |
| 13:10 | more is 49 . So we put the 49 down | |
| 13:12 | here . Now we multiply by the two . So | |
| 13:15 | dropped 02 times four is eight , two times two | |
| 13:18 | is four . We don't add these because that's from | |
| 13:20 | the previous step two times six is 12 . And | |
| 13:24 | then we have to add these numbers together . So | |
| 13:29 | the to come straight down nine plus eight is 17 | |
| 13:33 | because 10 plus eight is 18 . So nine plus | |
| 13:35 | eight is 17 carry the one , This makes 10 | |
| 13:38 | plus the four is 14 , carry the 14567 goes | |
| 13:43 | right here and then a one drops down here . | |
| 13:46 | Now look at all these numbers , where does the | |
| 13:47 | decimal go ? It certainly isn't in a line so | |
| 13:50 | we can't drop it down . What do we do | |
| 13:51 | ? We have two digits from after the decimal here | |
| 13:54 | and we have one digit after the decimal here . | |
| 13:56 | For a total of three digits after the decimal . | |
| 13:59 | So in our problem we must have in the answer | |
| 14:01 | three digits after the decimal . So the answer is | |
| 14:04 | 17.472 . Now we have not actually covered decimal positions | |
| 14:10 | past the hundreds place yet but really it's it's the | |
| 14:14 | same thing , it's the same concept here is the | |
| 14:16 | 10th place . Here is the 100th place . Here | |
| 14:20 | is the 1000th place . So what this means is | |
| 14:23 | this digit is worth 1 1/10 . Every digit here | |
| 14:27 | is worth 1 1/100 and every digit here is worth | |
| 14:30 | 1 1000 . It's worth a really tiny amount . | |
| 14:34 | The farther you go after the decimal , the digits | |
| 14:36 | mean less and less and less and less . So | |
| 14:39 | in real life sometimes we either round the numbers or | |
| 14:42 | we just throw the last few numbers away because they | |
| 14:44 | don't matter . You can have numbers after the decimal | |
| 14:47 | that go on and on and on forever . And | |
| 14:49 | those digits way at the end don't really mean very | |
| 14:51 | much because they're only worth maybe 11 millionth or 1 | |
| 14:55 | 100 million or something like this . So don't stress | |
| 14:58 | out if you have a lot of extra digits it | |
| 15:00 | looks like it's a big number but it's really telling | |
| 15:02 | you have 17 whole pieces and then This represents how | |
| 15:06 | close I am to the next hole . If I | |
| 15:08 | get to 999 after the decimal then I'll roll over | |
| 15:12 | to 18 the next hole number . So 17.472 is | |
| 15:17 | the final answer . All right , next problem . | |
| 15:19 | Let's take a look at 3.73 . And we're gonna | |
| 15:23 | multiply that by 077 . Now again , I'm not | |
| 15:28 | lining up the decimals . They only are lined up | |
| 15:30 | just because the columns are lined up with the numbers | |
| 15:34 | . All right , So what do I do I | |
| 15:36 | say ? Uh Seven times three I start here seven | |
| 15:38 | times 3 . 21 . So I put it one | |
| 15:41 | carry a 27 times 7 , 49 . Go up | |
| 15:44 | 50 51 carry the five , Seven times three , | |
| 15:50 | We go up 22 , 24 , 26 . means | |
| 15:55 | I just put a 26 down here . All right | |
| 15:58 | now , I gotta multiply by this seven so I | |
| 16:00 | have to drop a zero again . Seven times three | |
| 16:02 | is 21 . So let me switch probably switch colors | |
| 16:06 | seven times three is 21 . So again I'm gonna | |
| 16:08 | carry it to seven times seven is 49 50 51 | |
| 16:12 | carry the five . And then seven times three is | |
| 16:15 | 21 . Then we have 22 , 23 , 25 | |
| 16:19 | , Put the 26 right here . So I'm done | |
| 16:22 | multiplying by these now I do have another digit there | |
| 16:25 | . Um but multiplying by zero doesn't do anything . | |
| 16:29 | Let's go ahead and do it . But you'll just | |
| 16:30 | see if we're going to multiply by this digit . | |
| 16:32 | We have to not only add 10 we have to | |
| 16:35 | drop to zero and then zero times this . 00 | |
| 16:38 | times this . 00 times this is zero . So | |
| 16:41 | a leading zero like that before the decimal doesn't change | |
| 16:45 | anything . The one drops down one plus one is | |
| 16:50 | 26 plus one is 76 plus two is eight and | |
| 16:54 | two drops down . Where does the decimal go ? | |
| 16:57 | I have two digits here in two digits here . | |
| 17:00 | After the decimal for a total of four . So | |
| 17:02 | I have to put the decimal right here . one | |
| 17:05 | , digits should come after the decimal 287 21 So | |
| 17:10 | this is the 10th place . This is the 100th | |
| 17:14 | place . This is the 1000th place . This is | |
| 17:17 | called the 10/1000 place . It's worth even less 10 | |
| 17:21 | times less than what has come before it . So | |
| 17:23 | this one here has very little meaning in the number | |
| 17:26 | , it doesn't change the value of the number much | |
| 17:28 | because the farther you go off to the right after | |
| 17:30 | the decimal , the less and less the numbers mean | |
| 17:34 | okay , we only have two more problems and we're | |
| 17:40 | going to conquer The next to last one right here | |
| 17:43 | . What about 4.36 and will multiply it by 2.94 | |
| 17:51 | . Again , I'm not lining up decimals , I'm | |
| 17:53 | just lining up columns and the decimals happened to line | |
| 17:56 | up . So this is going to be a big | |
| 17:58 | multiplication problem . First we have to multiply by 44 | |
| 18:01 | times six is 24 carry the two . All right | |
| 18:06 | , three times four is 12 , then we go | |
| 18:08 | up 13 14 . Put a four carry the one | |
| 18:12 | , four times 4 is 16 . 1 more makes | |
| 18:14 | 17 . So the 17 goes there . Now we're | |
| 18:18 | done with the forward . Now we multiply times nine | |
| 18:20 | . We have to drop a 09 times 6 . | |
| 18:22 | 54 . Let me switch colors . 54 . Put | |
| 18:26 | a four , carry the five to the next column | |
| 18:29 | here . Nine times 3 , 27 . We add | |
| 18:33 | 28 29 30 31 32 32 means we carry a | |
| 18:37 | three nine times 4 , 36 . Then we add | |
| 18:40 | 37 38 39 39 goes right there . Now we're | |
| 18:45 | done with that . Now we have to multiply times | |
| 18:47 | the two . So I think what I'll do is | |
| 18:49 | I'll go to purple . Here . We have to | |
| 18:51 | drop two zeros in this guy right here , multiplied | |
| 18:54 | by 22 times 6 . 12 carry the one . | |
| 18:58 | two times 3 is six . It just goes right | |
| 19:00 | here Actually two times 36 we have to add the | |
| 19:03 | one , right ? We almost made a mistake . | |
| 19:04 | There has to be a seven like this . And | |
| 19:09 | then two times four is eight . We don't have | |
| 19:10 | to add any of those other numbers . That's from | |
| 19:12 | before . So 87200 Now we add all of these | |
| 19:16 | numbers together . What are we going to get ? | |
| 19:18 | We'll go back to black . The four just drops | |
| 19:20 | right on down . Four plus four is eight . | |
| 19:24 | That goes right here . 789 10 11 . We | |
| 19:27 | have to carry a one barely enough room right there | |
| 19:30 | to carry a 19 plus one is 10 . This | |
| 19:33 | is 10 . This is 10 plus 7 17 . | |
| 19:35 | So we have 17 . 1 more is 18 . | |
| 19:37 | Put in eight carry the one up here . +89 | |
| 19:41 | 10 , 11 , 12 . So we have 12 | |
| 19:44 | . We have a two here and we have a | |
| 19:45 | one here . So +128184 Now , we have to | |
| 19:49 | figure out where does the decimal go ? We have | |
| 19:52 | two digits after the decimal here . To from here | |
| 19:55 | , that means four . So we have to put | |
| 19:56 | the decimal right here for four positions After in our | |
| 19:59 | final answer , 128184 . So we have again , | |
| 20:04 | four digits tens , hundreds thousands . 10,000 . So | |
| 20:08 | again , this four means very , very little because | |
| 20:10 | it's so far toward the end , every number there | |
| 20:13 | is only worth a 10,000 . That's one out of | |
| 20:16 | 10,000 of something . It's not worth very much . | |
| 20:19 | And so that's the final answer . All right . | |
| 20:21 | We only have one . Last problem . And we're | |
| 20:24 | going to work it over here . Let's take a | |
| 20:26 | look at 9.91 and will multiply it by 1.2 . | |
| 20:31 | 1 point to notice the columns . Line up with | |
| 20:35 | the digits the decimals do not line up . All | |
| 20:38 | right . So first multiplied by 22 times one is | |
| 20:41 | what ? 22 times nine is 18 ? So we | |
| 20:45 | carry 12 times this . Nine is 18 . 1 | |
| 20:48 | more is 19 . So there's your 19 multiplied by | |
| 20:52 | one . We have to drop a 01 times one | |
| 20:54 | is 11 times nine is nine and one times nine | |
| 20:58 | is nine . And we add these numbers together . | |
| 21:02 | The two drops down eight plus one is 99 plus | |
| 21:06 | nine . You can think of it as nine times | |
| 21:08 | two which is 18 or you can count on your | |
| 21:10 | fingers . Nine plus nine is gonna give you 18 | |
| 21:13 | carry the one 9 10 11 means I have a | |
| 21:16 | one here and one here . So 11892 For the | |
| 21:20 | problem I have two digits after the decimal in this | |
| 21:23 | number and one digit after the number of here . | |
| 21:25 | For a total of three digits after the decimal . | |
| 21:28 | So the decimal has to go here three digits after | |
| 21:31 | the decimal 11.892 So what this means is if I | |
| 21:36 | take 9.91 of something , almost 10 of something and | |
| 21:39 | I multiply it just a little bit more than one | |
| 21:42 | then I should get I'm going to get a little | |
| 21:45 | bit more than 11 . Almost 12 , which makes | |
| 21:47 | sense because if I if you kind of round up | |
| 21:49 | this is very close to 10 and I 10 times | |
| 21:52 | one would be 10 . But I'm multiplying by just | |
| 21:54 | a little bit more than one , so I should | |
| 21:57 | get a little bit more than 10 and that's what | |
| 21:59 | I get . 11.892 So we have conquered multiplying decimals | |
| 22:04 | here using the rules , we ignore the decimal , | |
| 22:07 | we just do that at the end . Right ? | |
| 22:09 | And we of course now know what multiplying by decimal | |
| 22:11 | means from the previous lessons , I really need you | |
| 22:14 | to work all of these problems yourself . When you | |
| 22:17 | feel like you're getting the right answers , follow me | |
| 22:19 | on to the next lesson , we'll get a little | |
| 22:20 | more practice and wrap up the concept of multiplying decimals | |
| 00:0-1 | . |
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Learn to Multiply Decimals (Decimal Multiplication) - [15] is a free educational video by Math and Science.
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