Least Common Denominator

To find the least common denominator, simply list the multiples of each denominator (multiply by 2, 3, 4, etc.) then look for the smallest number that appears in each list.

Example: Suppose we wanted to add 1/5 + 1/6 + 1/15. We would find the least common denominator as follows...

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First we list the multiples of each denominator.

Multiples of 5 are 10, 15, 20, 25, 30, 35, 40,...

Multiples of 6 are 12, 18, 24, 30, 36, 42, 48,...

Multiples of 15 are 30, 45, 60, 75, 90,....

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Now, when you look at the list of multiples, you can see that 30 is the smallest number that appears in each list.

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Therefore, the least common denominator of 1/5, 1/6 and 1/15 is 30.

Use this method to find the least common denominator of these fraction pairs:

1. 5/18, 13/27 2. 3/14, 5/21 3. 11/16, 9/20 4. 6/25, 7/30