First you need to find the prime factors of each number involved.
Then you look for common factors and use them.
Finally you use all the factors that are not in common.
12 = 2 * 2 * 3
30 = 2 * 3 * 5
The common factors are 2 * 3
from the 12 you have a 2 left over and from the 30 there is a 5 left over. Therefore, the Least Common Multiple or Denominator is
LCD = 2 * 3 (common) * 2 * 5 the non-common
LCD = 2 * 3 * 2 * 5 = 60