Itself and any multiples of 9
3, 9, 15
That's an infinite list.
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" five multiples is wrong; there are infinitely many multiples for any number (except zero), not just five. To get those multiples, just use the definition of multiple. That is, you can do any of the following multiplications: 9 x 0 9 x 1 9 x 2 9 x 3 etc., also: 9 x (-1) 9 x (-2) etc.
9
9, 18, 27.
Itself and any multiples of 9
That's an infinite list.
18,36,54,72,90,
3, 9, 15
That's an infinite list.
That's an infinite list: All multiples of 9.
126, 252, 378 and so on.
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.
There are infinitely many multiples of 9 and 21 so it is impossible to list them. One of their multiples is 9*21 = 189.
18 is a multiple of 9. To be an LCM, it has to be compared to another list of multiples.