That really depends on what information is given.
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You can find a missing denominator if you know something that the fraction is equal to. Then you can find the missing denominator through cross multiplication.
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-- First, find the line in the middle of the fraction. -- Then look under the line. The number you find down there is the denominator.
Simply change the numerator and you will have another - different - fraction wit the same denominator.
Find a common denominator.