In mathematics, the rule of three is the metho of finding the fourth term of a mathematical proportion when the first three terms are known, that is, where the first term is in proportion to the second as the third is to the unknown fourth term. To find the fourth term, multiply the second and third terms (this is often called "taking the cross-product" or "cross-multiplying"), then divide their product by the first term. It was also known as the Golden Rule.