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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.

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• 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
(1 vote)
• so is it like the LCM
• 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
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• 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?