Ask the spectator to think of a number between 10 and 19 (these numbers included), then say to them to add the two digits and then subtract from the chosen number.
The result will always be 9.
Eg: 11; 1+1=2; 11-2=9;
10; 1+0=1; 10-1=9;
This force is also very useful to make a spectator choose a card that the magician wants.