The least common multiple of 12 and 20 is 60. The least common multiple is the lowest positive number that is a shared multiple of two or more positive numbers.
In this instance, the lowest multiples of 12 are 12, 24, 36, 48 and 60. Meanwhile, the lowest multiples of 20 are 20, 40, 60, 80 and 100. The lowest multiple that the two have in common is 60. Finding the least common multiple between two numbers, or among a set of more than two numbers, is important when adding and subtracting fractions, a process in which it's necessary to find the least common denominator.