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## 6th grade

### Unit 2: Lesson 5

Dividing fractions by fractions- Understanding division of fractions
- Dividing fractions: 2/5 ÷ 7/3
- Dividing fractions: 3/5 ÷ 1/2
- Dividing fractions
- Dividing mixed numbers
- Divide mixed numbers
- Writing fraction division story problems
- Interpret fraction division
- Dividing whole numbers & fractions: t-shirts
- Area with fraction division example
- Dividing fractions word problems
- Dividing fractions review

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# Understanding division of fractions

CCSS.Math:

Using a number line, we'll explain why multiplying by the inverse is the same as dividing. Created by Sal Khan.

## Want to join the conversation?

- OK, here we go with my question. I easily understand how to multiply and divide fractions. I have watched these videos over and over and still do not understand conceptually WHY I flip the reciprocal and multiply across to get the answer. Is there another source to read or watch to explain why these steps work and what is actually happening?(14 votes)
- This is a great question and I'm glad you are thinking at a deeper level in mathematics. The properties of numbers are useful for explaining why we flip the second fraction and multiply, when we divide fractions.

For b, c, and d nonzero, the problem a/b divided by c/d is the answer to the question "what number times c/d is a/b?" This is because of division's inverse relationship with multiplication (basic definition of division), but now applied to fractions.

Because any number times 1 is itself, d/c * c/d = dc/cd = cd/cd = 1, and factors can be grouped in any order (associative property of multiplication), we have

a/b = a/b * 1 = a/b * (d/c * c/d) = (a/b * d/c) * c/d.

So a/b * d/c is the answer to the question "what number times c/d is a/b?"

Therefore, a/b divided by c/d equals a/b * d/c.(3 votes)

- Its kinda confusing but im getting the hang of it thanks for the video!(5 votes)
- hi I just watched this and none of this made sense to me could really slow down the process(5 votes)
- You can click the gear button, and click the playback speed and select 0.50(2 votes)

- how is 6/9 changed into 2/3(5 votes)
- And sorry for the late comment(1 vote)

- I thought the anwer was 8/3 why is it not?(3 votes)
- If two fractions with a common denominator are divided, the common denominator is canceled out instead of kept (unlike addition or subtraction of fractions).

Note that for positive numbers, dividing by a number less than 1 gives a larger answer. Because 1/3 is less than 1, 8/3 divided by 1/3 is**greater**than 8/3, not equal to 8/3. So the answer 8/3 actually would not make good sense.

Another way to think of 8/3 divided by 1/3 is to answer the question “how many 1/3’s are in 8/3”? Clearly there are 8 of them, so the answer is 8.(4 votes)

- I understand how 8/3 / 1/3 is equal to 8; 8/3 is an improper fraction that is greater than 1. But if we divide 1/3 / 8/3, the answer is 1/8. How can we show that on a number line?(3 votes)
- Think about it. You are dividing 1/3, which is already a small number, into 8/3 parts. 8/3 is greater than 1, so your answer will be smaller than what you started out with. You end up with each part being 1/8.(4 votes)

- At :45 seconds in the video Sal said 8/3 divided by 3 but shan't it be 8/3 divided by 1/3(3 votes)
- Yes - This is a known error in the video. A correction box appears at about0:47telling you Sal meant divided by 1/3. Note: The correction boxes only appear if you aren't watching in full screen mode.(2 votes)

- how do you divide fraction without a number line(3 votes)
- With KCF (Keep, change, flip). For example, if you had 3/5 divided by 10/6, you would keep the first number(3/5) the same; change the sign(÷) to the opposite, in this case it is ×; then flip the second fraction(10/6), it will be 6/10. Now your equation should be 3/5 × 6/10. then you multiply across. You would do 3 × 6, which is 18. Then 5 × 10, which is 50. So your answer is 18/50, or 9/25 (simplified). Hope this helps!(4 votes)

- Can't you also divide 8 and 1 and 3 and 3 to get the first answer, and divide 8 and 2 and 3 and 3 to get the second answer?(5 votes)
- Actually, this method will work ONLY if you have common denominators. It's easier to do it the other way. But you can find common denominators, which cancel each other out, then the answer is the first numerator divided by the second numerator. For example,

2/3 ÷ 5/4 = 8/12 ÷ 15/12 = 8/15 over 12/12 or 8/15 (since 12 divided by 12 is 1)

You can also do it without common denominators, but then you're left with

2/5

----

3/4

which is another dividing fractions problem, so doing it without the common denominator creates an infinite loop of dividing fractions, and no one wants that!(0 votes)

- The video is kind of hard to understand :/(4 votes)

## Video transcript

Let's think about what
it means to take 8/3 and divide it by 1/3. So let me draw a
number line here. So there is my number line. This is 0. This is 1. And this is 2. Maybe this is 3 right over here. And let me plot 8/3. So to do that, I just need
to break up each whole into thirds. So let's see. That's 1/3, 2/3, 3/3,
4/3, 5/3, 6/3, 7/3, 8/3. So right over here. And then of course,
9/3 would get us to 3. So this right over here is 8/3. Now, one way to think
about 8/3 divided by 3 is what if we take this length. And we say, how
many jumps would it take to get there, if we're
doing it in jumps of 1/3? Or essentially, we're
breaking this up. If we were to break up
8/3 into sections of 1/3, how many sections would I have,
or how many jumps would I have? Well, let's think about that. If we're trying to
take jumps of 1/3, we're going to have to go 1,
2, 3, 4, 5, 6, 7, 8 jumps. So we could view
this as-- let me do this in a different color. I'll do it in this orange. So we took these 8
jumps right over here. So we could view 8/3 divided
by 1/3 as being equal to 8. Now, why does this
actually make sense? Well, when you're dividing
things into thirds, for every whole, you're
now going to have 3 jumps. So whatever value
you're trying to get to, you're going to have that
number times 3 jumps. So another way of thinking about
it is that 8/3 divided by 1/3 is the same thing
as 8/3 times 3. And we could either
write it like this. We could write
times 3 like that. Or, if we want to
write 3 as a fraction, we know that 3 is the
same thing as 3/1. And we already know how
to multiply fractions. Multiply the numerators. 8 times 3. So you have 8-- let me
do that that same color. You have 8 times 3 in the
numerator now, 8 times 3. And then you have 3 times
1 in the denominator. Which would give you 24/3, which
is the same thing as 24 divided by 3, which once
again is equal to 8. Now let's see if this
still makes sense. Instead of dividing by 1/3,
if we were to divide by 2/3. So let's think about what
8/3 divided by 2/3 is. Well, once again, this is
like asking the question, if we wanted to break up
this section from 0 to 8/3 into sections of
2/3, or jumps of 2/3, how many sections, or how many
jumps, would I have to make? Well, think about it. 1 jump-- we'll do this
in a different color. We could make 1 jump. No, that's the same
color as my 8/3. We could do 1 jump. My computer is doing
something strange. We could do 1 jump, 2
jumps, 3 jumps, and 4 jumps. So we see 8/3 divided
by 2/3 is equal to 4. Now, does this make sense in
this world right over here? Well, if we take 8/3 and we
do the same thing, saying hey, look, dividing by a
fraction is the same thing as multiplying by a reciprocal. Well, let's multiply by 3/2. Let's multiply by the
reciprocal of 2/3. So we swap the numerator
and the denominator. So we multiply it times 3/2. And then what do we get? In the numerator, once again,
we get 8 times 3, which is 24. And in the denominator, we
get 3 times 2, which is 6. So now we get 24 divided
by 6 is equal to 4. Now, does it make sense
that we got half the answer? If you think about the
difference between what we did here and
what we did here, these are almost the same,
except here we really just didn't divide. Or you could say you divided by
1, while here you divided by 2. Well, does that make sense? Well, sure. Because here you
jumped twice as far. So you had to take half
the number of steps. And so in the first
example, you saw why it makes sense
to multiply by 3. When you divide by a
fraction, for every whole, you're making 3 jumps. So that's why when you
divide by this fraction, or whatever is in
the denominator, you multiply by it. And now when the numerator
is greater than 1, every jump you're
going twice as far as you did in this first
one right over here. And so you would have to
do half as many jumps. Hopefully that makes sense. It's easy to think
about just mechanically how to divide fractions. Taking 8/3 divided by 1/3 is
the same thing as 8/3 times 3/1. Or 8/3 divided by 2/3 is the
same thing as 8/3 times 3/2. But hopefully this video
gives you a little bit more of an intuition of
why this is the case.