When fractions have different denominators, you need a shared denominator before you can compare them, add them, or subtract them. The common denominator is a number that both denominators divide into evenly.
Write a few multiples of each denominator. For example, if the denominators are 4 and 6:
The first number that appears in both lists is a common denominator. In this case, it is 12.
Usually, the smallest common denominator is the best choice because it keeps the numbers simpler. This is often called the least common denominator.
If you need to use the common denominator in a fraction problem, multiply the numerator and denominator by the same number so the value stays the same.
Make sure each original denominator divides evenly into the common denominator. If it does, your choice is valid.
If one denominator is already a multiple of the other, that larger number is the common denominator.
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