How are common denominator and a common multiple alike and different

A common denominator is when 2 fractions have the same denominator and a common multiple is when two multiples have at least 1 number alike. They are alike because they need to have a number in common and they are different because a common denominator has to do with fractions and a common multiple has to do with multiples. 

Maybe the answer is 20mm. all i really did was just multiply, which is obvious.