Common Denominators

The denominator is the number below the bar in a fraction, while the numerator is the number above. For example, in the fraction 2/5, the 2 is the numerator and the 5 is the denominator. A common denominator is a common multiple of the denominators of two fractions.

Tutorial

The following is a step-by-step example of how to find a common denominator for two fractions.

Solve: Find the common denominator for 4/8 and 7/9.


Step 1: Rewrite horizontal problems vertically.
Step 2: Identify the lowest number that is divisible by both 8 and 9. The lowest number that is divisible by both 8 and 9 is 72.

Step 3: To create equivalent fractions with a common denominator of 72, multiply 4/8 by 9/9 and multiply 7/9 by 8/8.

Step 4: Rewrite the equivalent fractions (with common denominators).

The common denominator is 72.