Looking at the numbers, we can see that the differences are 72, 36, and 18. Notice that they are each factors of the next by the same constant 2.
18 * 2 = 36
36 * 2 = 72
There, it means that 18/2 will be subtracted from 20.
20 - 18/2
= 20 - 9
= 11
The next number in the sequence would be 11. Hope this helps!
