Cross multiply only when you need to determine if one fraction is greater than another, or if you are trying to find a missing numerator or denominator in equivalent fractions.

What do you use cross-multiplication for?

You can use cross-multiplication to compare fractions and find out which is greater. When you do so, make sure that you start with the numerator of the first fraction. To find out which of two fractions is larger, cross-multiply and place the two numbers you get, in order, under the two fractions.

What is cross-multiplication rule?

In a cross multiplication method, the numerator of one fraction is multiplied to the denominator of another and the denominator of the first term to the numerator of another term.