You should have your original number back.

Let's use algebra to explain it.

1. Think of a number.	Let x = the original number.
2. Add 17.	Then we get x + 17
3. Double.	We have 2 (x + 17) or 2x + 34
4. Subtract 4.	Now we get 2x + 30
5. Double.	Now we have 4x + 60
6. Add 20.	We obtain 4x + 80
7. Divide by 4.	We get x + 20
8. Subtract 20.	We get x, the original number