Good old standard problem: Playing number game with your friend to select any of the number between 1 to 3. Whoever reaches 20 first, wins. You have to tell the strategy to win the game. Basically, you start with your position at 20 and recursively think the problem to ensure that you end up at 16 (problem specific) before so that your friend ends up at any number between 17 to 19 and so on solve for 16.