Their lowest common multiple is 63.
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LCM(9, 21) = 63 so the numbers are 63, 63*2 and 63*3.
18, 36, 54, 72, 90
180
Each common multiple of 9 and 10 occurs exactly once (in the list of common multiples), so there is no common multiple that occurs the most. All common multiples of 9 and 10 are the multiples of their least common multiple which is 90.
Multiples of 9 include 9, 18, 27 and so on. For them to be common, they need to be compared to another set of multiples.Itself and any other multiples of 9