The Ping Pong Puzzle

Three friends - A, B, and C, are playing ping pong. They play the usual way - two play at a time, the winner stays on the table, the loser lets the third one play. If you know that A played 10 matches in total, B played 15 matches in total, and C played 17 matches in total, who lost the second game?

A lost it. Since there have been (10+15+17)/2=21 games played in total, and each player never misses 2 games in a row, the only way for A to play just 10 games is if he plays the 2nd, 4th, 6th, etc. games, and every time loses.