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9 is a common factor of
9 and 36.
Here's why:
6 is not evenly divisible by 9
16 is not divisible evenly by 9 (it is divisible by 8)
All the other numbers are divisible by 9.
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Hope everything is clear; if you need any more explanation/clarification, kindly let me know, and I will comment and/or edit my answer. :)
