Question 108069: 13/15 - 11/20 =
Help again please, just need to no how to do these again. My mind is not of any help need proffesionals...
Answer by solver91311(24713)   (Show Source):
You can put this solution on YOUR website!  requires that you find a common denominator. It is usually most convenient to find the lowest common denominator.
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First begin with the prime factorization of 15, which is 3 * 5.
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Then find the prime factorization of 20, which is 2 * 2 * 5.
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Therefore, the lowest common denominator has two factors of 2, one factor of 3, and one factor of 5, i.e. 2 * 2 * 3 * 5 = 60.
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Now you can multiply each of your fractions by 1 in the form of some  where a = 60 divided by the denominator of the fraction.
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 so multiply the first fraction times  :
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Similarly,  , so 
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Now your problem reads:
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Since the denominators are now equal, you can add the numerators directly to get the requested sum:
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Just to show the convenience of finding the lowest common denominator, if you had just multiplied the two denominators together to get a convenient common denominator that wasn't necessarily the lowest, in this case 300, you would have had the following result:
 , and  is a rather messy fraction to have to reduce to lowest terms. I'll leave it as an exercise for you to prove that  .
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