You are given 4 cards, the operations +, -, x, / and brackets. Your task is to construct an expression using all 4 cards that evaluates to 24. Oh, and you have to find an answer in less than 1 minute. Initially this task is can be very challenging, but the more problems you try, the easier they become. From Cool Maths.
http://www.coolmath-games.com/0-make24/index.html