210, 420, 630, 840 . . .
LCM of 21 & 49 is 147, so all multiples of 147 are common multiples of 21 & 49.
there are infinite amount of common multiples.
The common multiples of 9 and 21 are 63, 126, 189 and so on.
Two common multiples of 21 and 15 are 105 and 315.
The common multiples of 3 and 21 are 21, 42, 63 and so on.
Common multiples of 9 and 10 are all of the multiples of 90.
Multiply 21 and 11 to get a common multiple. Since 21 and 11 have no common factors, that also happens to be the least common multiple.All other common multiples are multiples of this least common multiple.
Any pair of numbers has an infinite set of common multiples. The lowest common multiple of 9 and 21 is 63 and the others are multiples of this, e.g. 63, 126, 189, 252, 315, ...
There are 10 multiples of 6 in that range.
Multiples of 21 include 21, 42, 63, 84 and so on. For them to be common, they need to be compared to another set of multiples.
Odd multiples of 21.
Any multiple of 21.