There are 5 girls and 5 boys in a chess club. The club holds a round-robin tournament in which every player plays against every other player exactly once. How many games are payed in total?
There are ten players total ( does not matter that some are boys, some are girls)
each must play 9 other players 10 * 9 = 90 games
BUT each game involves TWO players...so divide by 2 to get 45 games must be played