Have everyone in your group pair up and face each other. Each person holds up 0 to 10 fingers behind their back. On the count of three, have them pull their hands from behind their backs. The first person to yell out the correct sum of all the fingers wins. Do best 2 of 3. Then the winners of each pairing play against each other until you have a champion.
See The Maths Game for a slightly more difficult variation.