The document explains how to add and subtract fractions with different denominators. It describes finding the least common multiple (LCM) of the denominators, which allows converting the fractions to equivalent fractions with a common denominator. It then demonstrates adding 2/3 + 3/9 and subtracting 5/10 - 3/20 as examples, showing the steps of finding the LCM, converting the fractions, and then adding or subtracting the numerators over the common denominator.