What is the lcm of 16 and 20

LCM stands for 'Lowest Common Multiple' which basically means "what's the smallest number that has the numbers in its multiplication table, excluding 1."*

So, the only numbers that are LCMs of 16 and 20 are 2 and 4. The lowest one is 2, so that's your answer.

* It's actually a little more complex than that, but I guess that was the easiest way to put it.

The least common multiple is 2

16 1,2,4,8,16

20 1,2,4,5,10,20