Glossary of 5th Grade Mathematical Terms - Math and Reading Help A common denominator is a multiple of the denominators of two or more fractions, so that the fractions can be more easily compared and worked with. For example, a common denominator for the fractions 3/10 and 4/5 is 20, since both 5 and 10 can be multiplied by another number to equal 20. The fraction 3/10 equals 6/20, while 4/5 equals 16/20.