On the first day, 10 matches were held, and thus 10 T-shirts were given out. The eleventh T-shirt was only given out on the eleventh day of the tournament, which means that Petya and nine other participants did not win a single match in the first ten days. This is only possible if these participants (let's call them the unlucky ones) did not play against each other during these days, meaning each of them played against the ten other participants (those who received T-shirts on the first day) and lost to all of them. Therefore, in the remaining days, the unlucky ones will play against each other. In particular, on the eleventh day, they will be divided into five pairs, and the winners of these pairs will receive T-shirts numbered 11 to 15.

## Answer

Also on the eleventh.
Frankin B.R.

Among 20 schoolchildren, a tennis tournament was held. Each participant played one match every day; in total, over 19 days, each played exactly once with all the others. There is only one tennis court in the school, so the matches took place in sequence. Immediately after their first win in the tournament, a participant received a branded T-shirt. There are no ties in tennis. Petya became the eleventh participant to receive a T-shirt, and Vasya the fifteenth. Petya received his T-shirt on the eleventh day of the tournament. On which day did Vasya receive his T-shirt?