Math Antics - Multiplying Fractions - Free Educational videos for Students in K-12 | Lumos Learning

Math Antics - Multiplying Fractions - Free Educational videos for Students in k-12


Math Antics - Multiplying Fractions - By



Transcript
00:05 you've probably noticed that we've been talking an awful lot
00:09 about fractions around here lately . Well now I'm gonna
00:11 tell you something you can do with fractions . That's
00:14 really easy . No , no , not juggling fractions
00:22 , although that is pretty easy for me . I'm
00:26 talking about something even easier than that . I'm talking
00:29 about multiplying fractions . Multiplying fractions is super easy .
00:34 In fact , it's easier than adding fractions and that's
00:36 why we're going to learn it first . The reason
00:39 it's easier is because fractions are really just division problems
00:42 and multiplication and division get along much better than addition
00:46 and division . Now , since fractions are division ,
00:49 that means if I have the problem 1/4 times two
00:52 thirds . It's the same as the problem one divided
00:55 by four times two , divided by three . That
00:58 means I have both multiplication and division in the same
01:01 problem . And because they get along so well ,
01:04 that means I can just rearrange our problem to look
01:06 like this . Now , it looks like to multiplication
01:10 problems that are being divided and it looks just like
01:13 a fraction . In fact , if we go ahead
01:16 and do the multiplication one times two equals two and
01:20 four times three equals 12 , then we do have
01:22 a fraction and it's the answer to our problem .
01:25 So what does all this mean ? Well , it
01:27 means that to multiply fractions , all you have to
01:30 do is multiply the top numbers and then multiply the
01:33 bottom numbers . And today there's your answer as always
01:37 , let's see an example or two . Let's try
01:40 this problem . Two thirds times 4/5 . Now we
01:44 could rewrite the problem like we just saw but that's
01:46 not necessary . As long as we remember the procedure
01:49 first , we know our answer is going to be
01:52 a fraction . So let's go ahead and write a
01:53 new fraction line for it . Next we multiply the
01:57 top number's two times four equals eight . So it's
02:00 the top number of our answer . And last we
02:03 multiply the bottom numbers three times five equals 15 .
02:06 So 15 is the bottom number of our answer .
02:09 There we have it to over three times 4/5 equals
02:13 8/15 . See how easy that was and fun to
02:18 even funner than video games . Okay , time for
02:22 another example , Let's try 6/11 times 7/8 . From
02:28 our multiplication table , we know that six times seven
02:30 equals 42 . So 42 is the top number of
02:34 our answer . And on the bottom we have 11
02:36 times eight , which is 88 . So 6/11 times
02:40 7/8 equals 42/88 . Oh , now some of you
02:46 might see that this answer could be simplified , but
02:48 we'll save simplifying fractions for another video . All right
02:52 , let's see . One last example here it is
02:55 one half times 4/3 times 3/5 . Wait a minute
03:00 . This has three fractions multiply together and the middle
03:03 one looks like an improper fraction because it's top numbers
03:06 bigger than its bottom number . Does our procedure work
03:09 for this problem to , yep , all we have
03:12 to do is multiply all the top numbers together and
03:14 then multiply all the bottom numbers together and we'll have
03:17 our answer and this will work no matter how many
03:20 fractions we have to multiply . So on the top
03:23 we have one times four is four and four times
03:26 three is 12 . And on the bottom two times
03:29 three is 6 and six times 5 is 30 .
03:32 That means our answer is 12/30 . So there you
03:36 have it multiplying fractions is easy because fractions are just
03:40 another way of writing vision problems . But remember there's
03:43 another way to think about fractions . We can also
03:46 use fractions to represent parts of things like half a
03:49 pizza for instance . But does it make sense to
03:52 multiply half a pizza by another half ? Actually it
03:56 does . If you're thinking of fractions as parts of
03:59 something , then multiplying fractions is really like taking part
04:03 of another part , for example , here's half a
04:07 pizza . You can see that if I take one
04:09 half of that half , Then I get 1/4 of
04:12 a pizza . And if we do the multiplication one
04:15 half times one half , you see that we do
04:18 get 1/4 . So that's why you can think of
04:21 multiplying fractions as taking part of another part . In
04:25 fact , sometimes especially in word or story problems .
04:28 You'll see multiplying fractions written using the word of instead
04:32 of times , they may ask what's 3/4 of two
04:36 thirds ? And now , you know , they just
04:38 mean what's 3/4 times two thirds . So whether you
04:43 think of fractions as parts of something or as division
04:46 problems , the procedure for doing the multiplication is exactly
04:49 the same . Let's do a quick review of what
04:52 we've learned . Multiplying fractions is even easier than adding
04:56 fractions . It's easy because order of operations says that
05:01 we can do the multiplication before the division . The
05:05 procedure for multiplying fractions is to multiply the top numbers
05:08 to get the answers , top number and multiply the
05:11 bottom numbers to get the answers . Bottom number .
05:13 If you think of fractions as parts of something ,
05:16 then multiplying a fraction by another fraction is the same
05:20 as taking apart of apart . And sometimes especially in
05:24 word problems , you might see the word of instead
05:27 of the word times , even though multiplying fractions is
05:31 so easy , it's a good idea to practice so
05:33 be sure to do the exercises and I'll see you
05:36 next time learn more at math antics dot com
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