To add or subtract fractions with different denominators, you first must find the least common denominator -- the smallest number that can be divided evenly by both denominators. The easiest way to find the least common denominator is to write the multiples of both denominators until you find a number they have in common. For example, to add the fractions 1/3 and 2/5, begin with some multiples of 3 (3, 6, 9, 12, 15) and some multiples of 5 (5, 10, 15). Youll find that 15 is the lowest number that both numbers divide evenly into. So,15 is the least common denominator of 1/3 and 2/5.