Pre-Algebra

How to solve

In a proportion the cross products are equal: a/b = x/c means a·c = b·x.

Steps

  1. Cross-multiply the two diagonals.
  2. Set the products equal.
  3. Divide to isolate the unknown.

Example

Solve 3/4 = x/20.

  1. Cross products: 3 · 20 = 4x.
  2. 60 = 4x, so x = 15.

Answer: 15

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