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### Course: Staging content lifeboat>Unit 14

Lesson 15: 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
• I think you need to multiply the two denominators that aren't the same and find the LCM
• So, fractions do not only represent equal parts of a whole, but are groups or things?
• 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.
• what if the two top numbers are bigger than the bottom number like this: 2/8+7/8=9/8
• so is it like the LCM
• what does it mean real world problem?
(1 vote)
• 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.
(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)
• Why did they have to make the spiders so creepy
(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)

## 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.