Fractions must share a common denominator in order to be added or subtracted. The common denominator is the least common multiple of all the denominators.

Fractions are generally presented with the numerator and denominator as small as is possible meaning there is no number, except one, that can be divided evenly into both the numerator and the denominator. To reduce a fraction to lowest terms, divide the numerator and denominator by their greatest common factor (GCF).