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The lowest common denominator (LCD) is also known as the least common multiple (LCM).

The simple way to find the LCD of 2 numbers is to make a list of the multiples of each number until you find a number which is in both lists. For example, find the LCD of 12 and 28:

12, 24, 36, 48, 60, 72, 84, ...

28, 56, 84, ...

The LCD is 84.

Sometimes you have to make long lists, and this method gets tedious.

The more technical method is to find the prime factorization of each number.

12 = 2x2x3

28 = 2x2x7

Notice that both numbers have 2x2, but we need a 3 to get a multiple of 12, and a 7 to get a multiple of 28. So you need 2 2's, a 3 and a 7:

The LCD is 2x2x3x7 = 84.

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Q: How do you calculate least common denominator?
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