Math Antics - Percents Missing Total - By Mathantics
Transcript
| 00:03 | Uh huh . Hi , welcome to Math Antics . | |
| 00:08 | In our last lesson about Percents , we learned that | |
| 00:10 | there are three main types of percent problems because there's | |
| 00:13 | three different numbers that could be missing . Those three | |
| 00:17 | numbers are the part the total and the percent . | |
| 00:21 | They're just the three variables or changeable numbers in the | |
| 00:24 | percentage equation . The fourth number is always 100 . | |
| 00:28 | Since that's what per cent means per 100 . In | |
| 00:31 | the last two videos , we learned how to solve | |
| 00:34 | problems where the part was unknown and where the was | |
| 00:37 | unknown . In this video , we're going to learn | |
| 00:39 | how to solve problems where the total is unknown or | |
| 00:41 | missing . With this type of problem . You'll be | |
| 00:45 | told what the percent is and you'll be told what | |
| 00:47 | part of the total you have . But you'll need | |
| 00:49 | to figure out what the total itself is . Here's | |
| 00:53 | an example of a problem like that . Your friend | |
| 00:55 | has a bag of marbles and he tells you that | |
| 00:58 | 20 of the marbles are red . If there's seven | |
| 01:01 | Red Marbles , how many Marbles does he have all | |
| 01:04 | together ? Okay . So how do you know that | |
| 01:07 | it's the total that's missing in this problem . Well | |
| 01:10 | the word all together is a big clue because it | |
| 01:13 | means almost the same thing as total . So if | |
| 01:16 | the question has words like all together or in all | |
| 01:19 | or total or whole or entire , those can help | |
| 01:24 | you know that you need to find the total . | |
| 01:27 | And another way that we can tell is by the | |
| 01:29 | numbers that we are given In this problem , we | |
| 01:32 | know that the % is 20 and we're also told | |
| 01:35 | that part of the marbles are red . So we | |
| 01:38 | know that the part is seven . So that means | |
| 01:40 | that it must be the total that's missing . All | |
| 01:44 | right then . So how do we figure out what | |
| 01:46 | the total is ? Well , using a little algebra | |
| 01:49 | which you don't need to know how to do right | |
| 01:51 | here , we can rearrange our percent equation like this | |
| 01:55 | . Mhm . Yeah . What this new form of | |
| 01:57 | the equation tells us is that if we take the | |
| 01:59 | part and multiply it by 100 and then we divide | |
| 02:03 | that by the 1% . We'll get the total . | |
| 02:07 | That seems simple enough . It's just two steps . | |
| 02:10 | Let's try it out on her word . Problem about | |
| 02:12 | the marbles . We know that the part that are | |
| 02:15 | red is seven . So step one is to just | |
| 02:18 | multiply that part by 100 seven times 100 is 700 | |
| 02:24 | . And in step two we take that 700 divide | |
| 02:27 | it by the percent . Which we're told is 20 | |
| 02:30 | . Okay , 700 divided by 20 . Well , | |
| 02:35 | we could use a calculator to divide but this doesn't | |
| 02:37 | seem too hard . So I'll just do the division | |
| 02:39 | no longer . 20 is too big to divide into | |
| 02:42 | the first digit seven . So we'll need to include | |
| 02:45 | the digit next to it as well . Now we | |
| 02:48 | ask how many 20's does it take to make 70 | |
| 02:51 | or almost 70 ? Well that would be three because | |
| 02:54 | three times 20 is 60 , 70 minus 60 leaves | |
| 02:59 | , 10 is the remainder . And then we bring | |
| 03:02 | down to zero . And then we ask how many | |
| 03:04 | twenties will divide into 100 ha ha five because five | |
| 03:08 | times 20 is 100 so that leaves no remainder . | |
| 03:12 | So 700 divided by 20 is 35 . And that | |
| 03:16 | means that the total number of Marbles is 35 . | |
| 03:20 | And in a problem like this you can always check | |
| 03:23 | your answer by making sure that the fraction of the | |
| 03:25 | part over the total would give you the correct percent | |
| 03:28 | . For example in this case you can make sure | |
| 03:31 | that the fraction 7/35 would really be 20% . Now | |
| 03:37 | that wasn't so tough , was it ? Let's see | |
| 03:39 | one more example to make sure you've got the procedure | |
| 03:41 | down before you try some on your own . The | |
| 03:44 | next problem says a high school marching band has 12 | |
| 03:48 | flute players . If eight of the band members play | |
| 03:55 | the flute , then how many members are in the | |
| 03:57 | entire band ? Okay , so the smaller part in | |
| 04:01 | this problem is 12 since there's 12 flute players And | |
| 04:05 | we're told that they make up eight of the band | |
| 04:09 | , so the is eight . Again , it's the | |
| 04:12 | total that's missing . And to find it , we | |
| 04:14 | just need to follow our two step procedure for step | |
| 04:18 | one . We multiply the part by 100 12 times | |
| 04:22 | 100 equals 12:00 for step two . We divide that | |
| 04:27 | 1200 by the percent , which is eight . This | |
| 04:30 | time I think I'll use a calculator to divide 1200 | |
| 04:34 | divided by eight equals 150 . Great . That means | |
| 04:38 | that the total number of band members is 150 . | |
| 04:42 | And again , you can always check your answer the | |
| 04:45 | way we did in the last example . All right | |
| 04:48 | . That does it for this lesson . Remember the | |
| 04:51 | key to getting really good at math is to do | |
| 04:53 | it yourself . Doing practice problems on your own will | |
| 04:56 | help you become a great problem solver . Good luck | |
| 04:59 | . Thanks for watching Math Antics and I'll see you | |
| 05:01 | next time learn more at Math Antics dot com . |
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