You need at least one more fraction in order to have a common denominator.
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The common denominator is 28.
There is none because the Least Common Denominator (or LCD) is the Least Common Multiple (LCM) of the denominators of a pair of fractions.
100
924
the common denominator for 9/8 and 7/20 is 40. (45/40 and 14/40)