A common multiple is a number that is a multiple of two or more numbers.
For example: Common multiples of 2 and 3 are 0, 6, 12, 18, ...
So, If one number is the multiple of the other, the bigger number will be the LCM
For example 6 is a multiple of 3. Therefore LCM of 3 and 6 is 6
hope his helps!