Used Open Tennis Balls used
Usually matches are started with 6 new balls for the server.
The rules dictate that the balls be changed every 9 games. But the first set of balls are changed after 7 games because they are used for the pre-match warm-up too.
For the remainder of the match, new balls are used every 9 games.
So the number of balls being used in a match is determined by the number of games that are played.
You can always calculate the number of sets of balls used as follows:
1. Add all the games played
2. Subtract 7 from the total.
3. Divide the remainder by 9. If the number is an integer, you're good and if non-integral, take closest lower integer.
4. Add 1 to the integral number you get. And voila!!! You have the number of sets of balls used in a match.
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