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When a pair of fractions both have the same denominator, you can add or subtract them by adding or subtracting their numerators and keeping the denominator the same. For example:

When a pair of fractions have different denominators, you need to get the denominators the same — that is, find a common denominator for both fractions. In some cases, you can do this by increasing the lesser denominator to the greater one. For example:

In other cases, you need to increase the denominators of both fractions. For example: