← Why the Formula Works · Lesson 3 of 4
Cross-multiplying
If 3/4 = x/20, the school rule says multiply across: 3 × 20 = 4 × x. The reason is that you are multiplying both sides by the same thing, 4 × 20, which keeps them equal and clears both fractions.
Then 60 = 4x, so x = 15. The two fractions are the same amount cut into different numbers of pieces: 3 out of 4 is 15 out of 20.
If 3/4 = x/20, what is x?
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