Puzzle: What should come in the place of question mark in the following series - 36 20 ? 8 6 5

Puzzle: What should come in the place of question mark in the following series - 36 20 ? 8 6 5 

Answer:

36-16 = 20

20-8 = 12

12-4 = 8

8-2 = 6

6-1= 5

Ans: 12