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### Course: Arithmetic > Unit 9

Lesson 5: Adding and subtracting fractions: word problems# Fraction word problem: spider eyes

Sal solves a fraction word problem about spider eyes. The fractions have common denominators. Created by Sal Khan.

## Want to join the conversation?

- what if the two top numbers are bigger than the bottom number like this: 2/8+7/8=9/8(11 votes)
- I think you need to multiply the two denominators that aren't the same and find the LCM(7 votes)

- So, fractions do not only represent equal parts of a whole, but are groups or things?(6 votes)
- True. If a class has 30 students and 28 are present at morning assembly then 28/30 are present. The whole in this case is all the students of the class combined. (This can be simplified to 14/15 though in a real case scenario it probably wouldn't be).

You can make such a set out of whatever you feel like: books you've read out of all the books you own. Capitals you've visited out of all the capitals in the world etc.(10 votes)

- what if the two top numbers are bigger than the bottom number like this: 2/8+7/8=9/8(2 votes)
- Its called a improper fractions. You can learn more about it in the 3rd grade course.(1 vote)

- so is it like the LCM(2 votes)
- what does it mean real world problem?(1 vote)
- are there other methods of doing it(1 vote)
- There is always more than one way to do
**any**type of problem. Always.(1 vote)

- is it just multiplying the numerator and the denomenator to get how many eyes are looking at you?(1 vote)
- You find the common denominator and then you multiply straight acrossed(1 vote)

- In a question i'm trying to answer its a mixed fraction. how would i solve that problem(1 vote)
- If the
*proper*fraction part of the mixed fractions have the same denominator—like for example 4 1/3 + 2 1/3—then you just add the whole numbers with each other and the numerators of the fractions with each other:

(4+2) = 6 and 1/3 + 1/3 = 2/3 to get the mixed fraction 6 2/3.

However, if you aren't that lucky, you first have to make the mixed fractions into lone proper fractions. If you have 4 1/3 + 2 1/6 you first multiply the whole numbers with the denominator of the fraction:

4 * 3 = 12 and then add it to the 1/3 to get 13 / 3.

2 * 6 = 12 and then add it to the 1/6 to get 13 / 6.

Then, we have to make both 13 / 3 and 13 / 6 into fractions with the same denominator, which we can do by multiplying 13 / 3 with 2 both on top and below. 13 / 3 = 26 / 6.

Now we can add these two fractions easily!

26 / 6 + 13 / 6 = 39 / 6.

And simplify it into 13 / 2.

And finally, if we want to make it back into a mixed fraction, we just see how many times 2 goes into 13... Which is 6.

And our answer as a mixed fraction is 6 1/2.(1 vote)

- but when the denominator is smaller than the numerator are we supposed to work them to make a mixed number as in a whole number and a fraction, or put the same denominator with the added numerator?(1 vote)
- you should make it a mixed number by converting it if you don't know how you can just ask mne(1 vote)

- so whenever you add fractions, you never have to change the denominator if they match?(0 votes)
- The denominator remains the same in addition when they match. In Multiplication the denominators are multiplied together.(2 votes)

## Video transcript

A common house
spider has 8 eyes. If a spider is looking at
you with its 2 front eyes and 3 of its other eyes,
what fraction of its eyes are looking at you? And just to show that we're
not making this stuff up, this is actually
pictures of spiders. And you see in every
one of these pictures they have eight eyes, even
this one's got other eyes right over here-- 1,
2, 3, 4, 5, 6, 7, 8. I never even realized this. This is actually quite creepy. But anyway, let's actually
focus on the problem. So it's looking at you
with 2 of its 8 eyes. So the 2 front eyes represent
what fraction of the eyes? Well, it represents
2 of the 8 eyes. You could look at
it right over here. You have 8 eyes. We have the 2 front eyes. That's 1 and 2 of the 8. And it says it's also looking
at us with 3 of its other eyes. So it's also looking at
us with 3 of the 8 eyes. And so you can
imagine that might be that one, that
one, and that one. So this is really
about adding fractions. This is about adding 2/8 of
something to 3/8 of something. So what fraction of its
eyes are looking at us? Well, it's still going
to be a fraction of 8. It's going to be still out
of 8-- 2 plus 3 over 8, which is just going to be
equal to 5/8 of the eyes. 5 out of the 8 eyes
are looking at us. And we see it right over
here-- 1, 2, 3, 4, 5. 5 of the total of 8
eyes are looking at us in a fairly creepy way.