A zero at the end.
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Use some method which you probably already learned, to find the least common multiple. All the other common multiples are multiples of this least common multiple, so you can multiply the least common multiple by 2, by 3, by 4, etc., to get additional common multiples.
2 and 5 are the prime factors that are common to all multiples of 10.
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.
The common multiples of 3,4, and 10 would be all the multiples of 60. 60, 120, 180, 240... all the way to 6000.
20 pinkgal333:20, 40, 60 are all common multiples.