The greatest common denominator is a number that all of the denominators of a set of fractions can be divided into evenly. To find this number, you will need to look at the denominators of all of the fractions in the set. Then, find the largest number that is common to all of the denominators. An easier way to do this is to list out all of the denominators and then find the smallest number that is divisible by all of the other numbers. For example, if the denominators showed were 4, 8, 12, and 16, you would list out all of the multiples of each denominator:

4: 4, 8, 12, 16, 20, 24… 8: 8, 16, 24, 32, 40… 12: 12, 24, 36, 48… 16: 16, 32, 48, 64…

The smallest number that appears in all four lists is 16, so it is the greatest common denominator.