Main content

## 4th grade

### Course: 4th grade > Unit 8

Lesson 2: Adding and subtracting fractions with like denominators# Subtracting fractions with like denominators

Sal subtracts 8/18-5/18. Created by Sal Khan and Monterey Institute for Technology and Education.

## Want to join the conversation?

- I dont like COVID-19 ;((41 votes)
- who doesn't honestly(9 votes)

- 4/5 divided by 2/5 equals 30/12 how is that wrong?(12 votes)
- 4/5 divided by 2/5 equals 4/5*5/2(because of reciprocal) which is 4/2 or 2. That's why it's wrong. Hope this helps!(31 votes)

- I have a hard time remembering and understanding what he says because he’s going way too fast…. Can there be a slower version please?(4 votes)
- Hi. Its Yagnesh here. if you don't understand what sal says, You need to do the following in order to slow down the video. you click on the setting button (looks like a wheel). then click on 'playback speed.'it will be normal. if you click on 0.75 or 0.5, it will become slower. Hope that helps.(9 votes)

- When subtracting fractions with the same denominators do we have to change the denominators?(4 votes)
- no the denominater stays the same(5 votes)

- 4/5 divided by 2/5 equals 30/12 how is that wrong(3 votes)
- why doesn't the denominators changes(3 votes)
- Becuase its subtracting with like denominators so they will not change they will stay the same because like means the same(2 votes)

- How is it possible to have a negative fraction? If you think about fractions as pieces of pie, wouldn't it be impossible to have negative pieces of pie?(3 votes)
- I understand your reasoning about that, but it doesn't quite work, with that logic you cant have negative whole number either, because you cant have a negative whole pie.

think of it like this; a fraction is a part of a number, right? well, you can have a negative part of a number as well as a negative number. look at this number line-

-1___________________0____________________1

something has to go in between those spaces!

hope this helps!(3 votes)

- What is it if the fraction 16/12 divided by 3/12 ? Will it work?(3 votes)
- You can absolutely do this. When dividing fractions there are a few methods you can work out. The problem you gave would have an answer of 16/3.

Here is a khan academy video on working out a dividing fractions problem: https://www.khanacademy.org/math/arithmetic/fraction-arithmetic/arith-review-dividing-fractions/v/dividing-fractions-example&sa=U&ved=2ahUKEwj0i_rWzOjoAhVnknIEHa_cAtEQFjAAegQIAhAB&usg=AOvVaw2M_V5C7X7Fe5TxqTpMGmeP(1 vote)

- at 1.33 what is the colored boxes soppost to mean?(2 votes)
- He is visually showing how to subtract those fractions. There's 18 boxes and he colors in 8 of them to represent 8/18. Then he gets rid of 5 of them to show the subtraction. and how there is 3 boxes left, or 3/18 left.(3 votes)

- What about subtracting numbers with different denominators? Could you describe it carefully?(3 votes)
- You would find a common denominator. You do that by finding a number that is a factor of both of the numbers in the fraction. If I were you, I would look up a Khan Academy tutorial on it.(2 votes)

## Video transcript

We're asked to subtract and
simplify the answer, and we have 8/18 minus 5/18. So subtracting fractions
is very similar to adding fractions. If we have the same denominator,
the denominator in the difference is going
to be the same as the denominators in the two
numbers that we're subtracting, so it's
going to be 18. And our numerator is going to be
the difference between the numerators. So in this case, it is 8 minus
5, and this will be equal to 3 over 18, which is the answer,
but it's not completely simplified, because both 3 and
18 are divisible by 3. So let's divide them
both by 3. So you divide 3 by 3, you divide
18 by 3, and you get 3 divided by 3 is 1. 18 divided by 3 is 6,
so you get 1/6. And just to see this visually,
let me draw 18 parts. Let me draw 18 parts here. So it might be a little bit
of a messy drawing. I'll try the best I can. So let me draw six in
this direction. So that is three right there. We have another three,
so that's six parts. And then let me split this
into three columns. So there we go. We have 18 parts. Now 8/18 is equal to one,
two, three, four, five, six, seven, eight. That's 8/18. And now we want to subtract five
of the eighteenths, so we subtract one, two, three,
four, five. Now, what do we have
left over? Well, we have three of the
eighteenths left over, so you have that right there. You have three of the
eighteenths left over. Now, if you turn three of the
eighteenths into one piece, how many of those bigger
pieces do you have? This is one of those
big pieces. Now, where are the other ones? Well, this is another big
piece right here. This is another big piece
right here, another one, another one, and another one. If you had 18 pieces and you
merged three of the pieces into one, then you actually end
up with only six pieces. You end up with six pieces. Hopefully, you see that each row
is one of the pieces now, and the blue is exactly one
of the six, so 3/18 is the same as 1/6.