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Math Videos 43 videos

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Math 5: Dividing Fractions by Whole Numbers 76 Views


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Description:

Dividing fractions is an important skill to have, but if anyone ever tried to hand us 5/24 of a pizza, we'd probably just drop it on the floor and steal their slice.

Language:
English Language

Transcript

00:02

[Dino and Coop singing]

00:12

We all know that fractions are parts of wholes, like slices of a cake. [Woman looks excited by cheesecake]

00:15

But is it possible to divide these fractions up even further?

00:19

Absolutely.

00:20

Although we don't recommend handing out one tenth of a slice of cake at a birthday party.

00:24

Unless you like seeing kids cry, you monster.

00:27

Let's think about this in terms of an example.

00:29

Carly has five pieces of a pizza left, from a pizza that was originally cut into eight

00:33

equal slices.

00:34

She wants to divide these five slices between herself, her brother, and her sister. [Carly looks confused]

00:38

So how much of the original pizza should each sibling get?

00:41

If Carly happened to have an extra set of twin siblings, that'd be pretty easy: everyone

00:45

would get one slice each. [Extra siblings appear]

00:46

However, we can't just make up twins to make our calculations easier––trust us, we've

00:50

tried it before––so we'll have to do some math.

00:53

Let's think in terms of fractions.

00:55

We have five slices of a pizza that was originally cut into eight slices, so we've got five eighths

01:00

of the original pizza.

01:02

And, since there's a total of three siblings, we want to divide this fraction by three.

01:06

Uh-oh.

01:07

We've never seen a division sign near a fraction before.

01:09

Does that mean Carly and her siblings should abandon the pizza and flee from the house, just

01:13

in case they make a mathematical mistake?

01:15

Well, they could, or they could just learn about reciprocals. [Carly and her sibling looks scared and run out the house]

01:18

…Uh…we're sure they'll be back…soon…ish…

01:20

Anyway, a number's reciprocal is the number flipped upside down.

01:26

With a fraction, that means the original numerator becomes the denominator…

01:29

…and the original denominator becomes the numerator.

01:32

If you're getting dizzy, then you're doing something wrong…the reciprocal's the one

01:36

who actually has to flip upside down.

01:38

You can stay right where you are. [Woman falls out of office chair]

01:40

When we divide fractions, we're really multiplying by a reciprocal.

01:43

We take the second number in our expression, flip it upside down, and then multiply everything

01:47

together.

01:48

Let's see how this works with our particular example.

01:51

We're dividing a fraction by three, so since three's the second number, we need to find

01:55

it's reciprocal.

01:56

Remember, all whole numbers are really just the whole number over one…

02:00

…so the reciprocal of three is one third.

02:03

Now all we've gotta do is some familiar fraction multiplication… [Dino points to a blackboard]

02:06

…and we find our answer: five twenty-fourths.

02:09

Now the only difficult part is cutting those slices up into even skinnier slices.

02:13

We hope Carly's got some precise measuring equipment…

02:16

Actually, we hope Carly comes back…has anyone seen her? [Carly's mum looks worried]

02:19

Or her siblings…?

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