How to guess the number someone is thinking?

Once your friend has thought of a number, do the following:
1. Ask him or her to multiply the chosen number by 2.
2.Choose an even number to use yourself. Ask your friend to add this number to the one in his head.
3.Tell him/her to divide the new number by 2.
4.Tell your friend to subtract his original number from the equation.
5."Guess" the number.
:)