of 20 and 36
It is if you're looking for the least common multiple, which I assumed.
List the multiples of 20 and 80. You see that 180 is the first multiple common to both. There is an infinite numbers of common multiples, but 180 is the least common multiple (LCM).
20, 40, 60, 80, 100, 120, 140, 160, 180
36, 72, 108, 144, 180