Sebastian won Oscar a tennis set 6-3. Five games were won by the player who did not serve.
Who served first?
Whoever served first served 5 games and the other player served 4.
Imagine who served for the first time won x of the games that served e Y from the rest of the games. The total number of games lost by the player who served them will be 5 - x + Y. This will give a result of 5 (as we have been told that who did not serve won 5 games), x = y, and the first player won a total of 2x games. Given the only Sebastian won an even number of games, he was the first to serve.