1. Well, to cross multiply them you multiply the numerator in the first fraction times the denominator in the second fraction, then you write that number down. ... The reason we cross multiply fractions is to compare them. Cross multiplying fractions tells us if two fractions are equal or which one is greater.
2. Cross multiplying fractions to find a missing numerator or denominator for equivalent fractions works because it is a shortcut for rearranging to isolate the variable.
