Multiplying Fractions - By Anywhere Math
Transcript
00:0-1 | you and your friends show up a little late to | |
00:02 | a birthday party and by the time you get there | |
00:05 | there's only a half a pizza left , the three | |
00:08 | of you split it evenly and each eat a third | |
00:10 | of the half of pizza . How much pizza did | |
00:14 | each of you eat ? Welcome to anywhere . Math | |
00:34 | . I'm Jeff , Jacobson . And today we're gonna | |
00:36 | talk about multiplying fractions . Okay , so let's talk | |
00:40 | about that pizza , you show up a little late | |
00:44 | and unfortunately there's only a half a pizza left , | |
00:46 | which is a pretty big bummer . Uh let's show | |
00:49 | what that looks like visually . Well , let's draw | |
00:52 | a picture of a pizza . So here's the whole | |
00:56 | pizza , by the time you get there , there's | |
00:59 | only half left . Okay , So here's the half | |
01:06 | of a pizza that's left that you and two other | |
01:10 | friends have to split . So each of you have | |
01:14 | a third of that half . So what I'm gonna | |
01:17 | do is I'm gonna make that I'm gonna try to | |
01:20 | make it as even as possible . So each of | |
01:24 | you have a third of that . So the question | |
01:27 | is well how much pizza did you actually eat ? | |
01:30 | And it's relative to the whole pizza ? So this | |
01:34 | is maybe I'll do it and I'll do it in | |
01:37 | blue . Each of you had a third 1/3 of | |
01:44 | a one half of the pizza . So how much | |
01:47 | total is that ? Well you see these are split | |
01:50 | into thirds here of the half . We need to | |
01:54 | make the same size pieces here . So I'm gonna | |
01:57 | do the same thing and split that like that . | |
02:03 | Now that they're all the same size pieces , we | |
02:05 | can actually figure out what one third of one half | |
02:13 | pizza is . Okay that's the question . Well what | |
02:17 | is one third of one half ? Well this is | |
02:21 | equal to one Out of six . There's 123456 total | |
02:27 | pieces of that size in the whole pizza . You | |
02:31 | only ate you and your friends each only ate one | |
02:33 | of them . So the answer is yeah one . | |
02:38 | Okay . Six . Now let's look at how can | |
02:42 | we actually get there uh With just the math without | |
02:46 | actually drawing a picture . If you remember of inward | |
02:50 | problems . That means multiplication , so I can rewrite | |
02:53 | this as one third times one half . Yeah . | |
02:59 | And we know it's gotta be 16 And if I | |
03:03 | look well , how would I get 16 ? Uh | |
03:05 | from this problem ? Well one times one is one | |
03:10 | and three times 2 is six . So if you | |
03:14 | notice all we had to do was multiply straight across | |
03:17 | numerous times numerator and denominator times denominator . Let's look | |
03:21 | at another example . Okay so example one , we're | |
03:24 | gonna start off pretty simple . 1/5 times one third | |
03:28 | . If you remember from that pizza problem , we | |
03:32 | said to multiply fractions , you don't need common denominators | |
03:36 | that's adding and subtracting . We don't care what the | |
03:38 | denominators are . Uh we just multiply straight across . | |
03:42 | So one times one is one And five times 3 | |
03:47 | is 15 . Now when you're working with fractions we | |
03:51 | always want are answers in simplest form . Uh in | |
03:54 | this case 1 15 is the simplest form . Yeah | |
03:59 | . So I'm done . Let's look at another example | |
04:03 | . All right , here's example number 289 times 34 | |
04:06 | So like we said before , you can just multiply | |
04:10 | straight across . So if I do that eight times | |
04:12 | three is 24 . nine times 4 is 36 , | |
04:17 | 24/36 . But hold on a second , I mentioned | |
04:23 | that when you deal with fractions , you always want | |
04:26 | your answer . In simplest form . This is not | |
04:30 | in simplest form and simplest form means that there aren't | |
04:34 | any other common factors besides one with a new range | |
04:38 | of denominator , 24 and 36 do have other common | |
04:42 | factors Between them between besides one . Ah If you | |
04:48 | look obviously they're both even numbers . So too is | |
04:51 | a common factor . Uh huh . six is also | |
04:54 | a common factor . Um 12 is also a common | |
04:59 | factor , so there's quite a bit of common factors | |
05:01 | . So what we need to do is simplify this | |
05:04 | and to do it quickly . All you need to | |
05:07 | do is basically if you can figure out what the | |
05:10 | greatest common factor is then you can simplify it in | |
05:14 | one step . If you can't figure out what that | |
05:17 | is then you can all you can do any common | |
05:19 | factor you want . You just , it just might | |
05:22 | take you a few more steps . So 12 is | |
05:25 | the greatest common factor of 24 and 36 . So | |
05:29 | if I divide The numerator and denominator by 12 , | |
05:34 | I get 2/3.24 divided by 12 is to 36 , | |
05:40 | divided by 12 is three . And if you look | |
05:43 | the reason I can do that is because all I | |
05:47 | did was essentially Divide This fraction by one . I | |
05:53 | didn't change the value of it , I just change | |
05:55 | what it looked like . 24/36 Is equivalent to 2/3 | |
06:02 | . The only difference is this has a lot more | |
06:05 | pieces but they're skinnier . Like think of a pizza | |
06:08 | again There's 36 pieces in this in this pizza . | |
06:13 | Uh but they're really small . So this is 24 | |
06:16 | hours at 36 here , there's only three pieces in | |
06:19 | the pizza but they're much bigger . Uh their equivalent | |
06:22 | the amount of it is still the same . So | |
06:25 | dividing by one doesn't change anything divided by one is | |
06:28 | still the same . Just like anything multiplied by one | |
06:31 | is still the same . So these are equivalent . | |
06:33 | Now this is not the best way to solve this | |
06:37 | problem . I want to show you how to do | |
06:41 | this problem much better uh and much easier . So | |
06:46 | let's solve this problem again . Uh Using a different | |
06:49 | method instead of simplifying at the end , I'm gonna | |
06:52 | show you how you can simplify at the beginning . | |
06:56 | Now first I'm gonna show you something and you don't | |
06:59 | really need to write it down . Just follow along | |
07:02 | and see if you can understand what I'm doing . | |
07:04 | So eight times 3 , I can write this as | |
07:08 | eight times 3/9 times four . Right ? We're multiplying | |
07:14 | , that's essentially what we do , right ? Numerous | |
07:16 | has numerator , denominator times denominator . So that's okay | |
07:18 | . That's that's fine . I can also , since | |
07:20 | we're multiplying I can change the order . Community of | |
07:23 | property says I can change the order when I'm multiplying | |
07:26 | . So that can become uh three times 8/9 times | |
07:32 | four . That's okay . Now , I can also | |
07:36 | break some of these uh introduce apart into a product | |
07:41 | of their factors . So instead of three times eight | |
07:46 | , I can write it as three times . Let's | |
07:50 | do two three times 2 times four . Well , | |
07:57 | three times eight is 24 3 times two is six | |
07:59 | times four is 24 . It's the same . I | |
08:01 | just broke that eight into two times four . So | |
08:04 | I'm all right there , same thing here with the | |
08:07 | nine . I'm going to break it up three times | |
08:10 | three uh times four . Okay , then I'm running | |
08:17 | a little bit out of room , so I'm gonna | |
08:19 | go over here ah I can now break them apart | |
08:24 | into fractions again , kind of basically what it was | |
08:28 | here , go in the opposite direction . So now | |
08:30 | I'm gonna make it 3/3 . Right ? That by | |
08:34 | itself times to over three times 4/4 . I can | |
08:41 | do that because essentially it's the same thing , right | |
08:44 | ? Three times two times four Here , three times | |
08:47 | 3 times four here . Okay . Um Now hopefully | |
08:52 | you notice something . Well , what is three times | |
08:55 | three ? I'm signed up three times 3 . 3/3 | |
08:58 | or three thirds . Well , that's just one times | |
09:02 | two thirds times 4/4 is the same as 14 forces | |
09:08 | one ? Well , one times two thirds is two | |
09:10 | thirds , two thirds times one is two . All | |
09:14 | right . Yeah . Which is what we got before | |
09:19 | . All we did was we tried to break uh | |
09:24 | these numbers apart into factors so that we could find | |
09:29 | common factors . So we know that eight and four | |
09:34 | Had a common factor of four here . Right ? | |
09:38 | Which , let us kind of cancel it out . | |
09:40 | It became one . Uh we also found that three | |
09:44 | and 9 had a common factor of three . So | |
09:49 | the same thing , we can basically kind of cancel | |
09:52 | that out and that becomes one And then what we're | |
09:55 | left with is just 2/3 . You will definitely not | |
09:59 | need to show all of this . Okay , let's | |
10:02 | show you one word . One more way of how | |
10:04 | to do essentially this much quicker and simpler , but | |
10:08 | it's important to know where that comes from . Okay | |
10:11 | , so let's try it one more time . Alright | |
10:13 | , Let's try this problem one more time . Um | |
10:16 | and now we're going to use the rule that is | |
10:19 | so so so important . And that is to always | |
10:22 | try to simplify before you multiply . Always try to | |
10:28 | do that first . It's going to make your life | |
10:30 | a whole lot easier . So I'm looking for common | |
10:34 | factors between any numerator and any denominators . We don't | |
10:39 | go common factors in the numerator by themselves or uh | |
10:44 | sorry , enumerators by themselves or denominators . It's got | |
10:47 | to be a numerator with a denominator so that you | |
10:50 | set up that situation where it becomes one . So | |
10:53 | like we said before eight and four have a common | |
10:55 | factor of four . So four sign eight divided by | |
11:00 | four becomes too . So I crossed out the eight | |
11:04 | and right to four divided by four becomes 1 . | |
11:10 | nine and 3 also have common factors . It's three | |
11:14 | , So nine divided by three is three and three | |
11:18 | divided by three is 1 . Now that I've simplified | |
11:22 | it before , I multiply , it's very easy to | |
11:25 | times one is two and three times one is three | |
11:30 | and there's my answer . It's already in simplest form | |
11:34 | because I simplified here first . If you do all | |
11:37 | your simplifications , hear your answer will automatically already be | |
11:41 | in simplest form . Okay ? Uh Here's some problems | |
11:43 | to try on your own . All right . Here's | |
11:51 | our last example example 31 half times two and 3/4 | |
11:56 | two and three forces a mixed number . Uh So | |
11:59 | what you want to do first is any time you're | |
12:02 | multiplying fractions with mixed numbers or mixed numbers with mixed | |
12:05 | numbers , Change it to an improper fraction . Uh | |
12:09 | And the reason is you could leave it like this | |
12:12 | but it's going to be much more complicated . You | |
12:14 | have to use the distributive property and it's kind of | |
12:17 | a pain . So just trust me , change it | |
12:20 | to an improper fraction first . So one half times | |
12:25 | two and 3/4 . If you remember how to convert | |
12:27 | it to an improper fraction , I just do numerator | |
12:30 | times the whole number . So four times two is | |
12:32 | eight plus , the numerator is 11 . The denominator | |
12:38 | stays the same , that becomes 11 force . Okay | |
12:42 | , now I'm ready to go . Uh and like | |
12:44 | we said earlier , always try to simplify first . | |
12:47 | So you're looking , are there any common factors between | |
12:50 | the numerator and denominator that I can simplify ? And | |
12:53 | here there isn't Don't do two and four . Remember | |
12:57 | ? Those are both denominators , you can't simplify uh | |
13:01 | anything that's both . Denominators are both numerous has to | |
13:04 | be numerator and denominator . So there isn't anything to | |
13:07 | simplify . So I just multiply straight across one times | |
13:11 | 11 is 11 and two times four is eight . | |
13:15 | Now I can leave it as a as an improper | |
13:18 | fraction . That's in simplest form . Or I can | |
13:21 | change it back To a mixed No . eight goes | |
13:25 | into 11 . Once there would be three left over | |
13:30 | . 3 , 8 left over . Okay , so | |
13:34 | those are both uh acceptable answers unless your teacher says | |
13:39 | they want something specific . Okay , here's some to | |
13:42 | try on your own . As always . Thanks for | |
13:51 | watching . And if you like this video please subscribe |
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