This one is a favorite of mine. Two spectators each hold out any number of fingers on one of their hands. (E.g. one might put out 3 fingers and the other, 2.) A third spectator tells the magician, who is looking the other way, the total (in this example, 5). The magician immediately announces that spectator #1 is holding out 2 fingers, and #2 is holding out 3. Just lucky? No, because you play 2 more rounds and the magician is correct each time! How can she possibly know this? One of the spectators is a confederate. He holds out 3 fingers in round #1, and in subsequent rounds, however many fingers spectator #2 held out the last round. So, e.g., in our above example, spectator #1 is the confederate. When you hear “5”, you know that #1 has 3, so #2 must have 2. In round 2, suppose the total is 6. You know that spectator #1 has 2 (same as #2 had the previous round), so #2 must have 4!
I like that this trick needs no props, and it hones subtracting in your head quickly. For a little more challenge, let the 2 people each use 2 hands. Good luck!