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## Arithmetic (all content)

### Unit 5: Lesson 11

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.

## 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.