In a round-robin chess tournament every player plays one game with every other player. Five participants withdrew after playing two games each. None of these players played a game against each other. A total of 220 games were played in the tournament. Including those who withdrew, how many players participated

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  26 players to start; 21 players after 5 withdrew

Step-by-step explanation:

We are told that the withdrawing players did not play against each other, so the total number of games they played was 5×2 = 10. Then the number of games played by the remaining players was 220 -10 = 210. When n players play each other, they play a total of n(n -1)/2 games. Here, that total is 210, so we have ...

  n(n -1)/2 = 210

  n^2 -n = 420 . . . . . . . . .multiply by 2

  (n -1/2)^2 = 420.25 . . . . add 0.25 to complete the square

  n = 1/2 +√420.25 = 0.5 +20.5 = 21 . . . . . square root and add 1/2

The number of participating players after 5 withdrew was 21. There were 26 players to start.

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step-by-step explanation: