When you ask this type of question, think of the parttern of it.
20,22. Well you added 2. 22,10. You subtracted 12. 10,17. You added 7.
17,13. You subtracted 4. So that was a patteren.
Now, a pattern is something that repeats itself.
Now the beginnging was add 2. So add 2 of 13 and you will get 15.
Answer: 15