Effect:
First you ask the spectator to think of any number, the ask the spectator to multiply it by two. After that, ask them to add any number but tell you the number they want to add, then divide it by two and subtract the number they thought of first. Then you tell them the final answer that they got.

Method:
You take the number the spectator adds and divide it into half.

Example:
10 x 2 + (any number,but in this case it's 4) divide by 2 and -10
=2
4 divided into half is 2