Because the common multiples of any two or more numbers are infinite.
Yes. Any pair of numbers has an infinite number of common multiples.
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, ...
20 and any multiple of 20. Any two multiples of 20 that differ by 20.
Common multiples of any set of numbers can be found by multiplying the numbers together.
Any that are multiples of 24.
A common multiple is used to compare the multiples of two or more numbers - a single number cannot have any common multiples.
The multiples of 2 and 24 include any multiple of 24. The multiples of 22 and 4 include any multiple of 44.
This cannot be determined because the common multiples of any two or more numbers are infinite.
Any pair of prime numbers
A single number cannot have any common multiples - you need two or more numbers to compare.
This cannot be answered because the common multiples of any two or more numbers are infinite.