How are common denominator and common multiples alike and different

A common denominator is when the denominators (bottom number) of two fractions are the same. Fractions must have a common denominator to be added or subtracted.
A common multiple is when two numbers can be multiplying to the same number. Ex) A common multiple of 3 & 4  is 12
       A common multiple of 6 & 2 is 18
You would need to know the common multiples of numbers in order to make fractions have a common denominator.

It is 2 out of 8 she will pick green and 3 out of 7 she will pick green but red and green would be 5 out of 5 so 50% chance