I know what you are thinking

1. Think of a number... any number
2. Add 10 to it
3. Multiply by 4
4. Subtract 10
5. Divide by 2
6. Subtract 5
7. Divide by 2
8. Subtract original number
9. Add 20
10. Multiply by 4

You are left with 100 :)

The basics of any trick like this is to get rid of unknown. Once that is done everything else is just a show. In the example above the unknown (your number at line 1) is cancels itself (in line 8). What we are left with is fairly simple arthematics.

These problems can be made as long as possible or as short as possible. The shortest being:
1. Think of a number
2. Subtract the number from it self

You are left with zero :)

Another shortest one
1. Think of a number (other than zero)
2. Divide it by itself

You are left with one :)