Math CentralQuandaries & Queries


Subject: cross multiplication
Name: Kayla
Who are you: Other

Why does eight over twelve compared to one half work when you use cross multiplication.

Hi Kayla,

I am going to look at two different fractions. Suppose you want to compare 2/3 and 5/8 to see which is larger. It would be easy to compare them if they had the same denominator so I am going to use a common denominator (in this case 3 x 5 = 24).

2/3 = 5/8 = 5/8

5/8 = 5/8 = 5/8

Since 16 > 15 you can conclude that 2/3 > 5/8

Now look at the second fractions in the two lines above, 5/8 and 5/8. The denominators are equal and the numerators are the result of cross multiplying 2/3 an 5/8.

I hope this helps,

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