The only thing that goes into 31 is one... 18 has 1,2,3,6,9,18 (3 multiples) and 20 has 1,2,4,5,10,20 so really the closet common multiple you have is 2
What are the first three common multiples of 31, 18, and 20?
I can't find the first three multiples, but their LCM is 5580.