
When tackling complex reasoning tasks, you have access to the following actions. Use them as needed to progress through your thought process.

[ASSESS]

[ADVANCE]

[VERIFY]

[SIMPLIFY]

[SYNTHESIZE]

[PIVOT]

[OUTPUT]

You should strictly follow the format below:

[ACTION NAME]

# Your action step 1

# Your action step 2

# Your action step 3

...

Next action: [NEXT ACTION NAME]


system
Among 20 schoolchildren, a tennis tournament was held. Each participant played one match every day; in the end, over 19 days, each played exactly once against everyone else. There is only one tennis court at the school, so matches were played sequentially. Immediately after their first win in the tournament, a participant received a branded T-shirt. There are no draws in tennis. Petya was 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?

Present the answer in LaTex format: \boxed{Your answer}
user