13.236. The master is giving a simultaneous chess exhibition on several boards. By the end of the first two hours, he won $10 \%$ of the total number of games played, while 8 opponents drew their games with the master. Over the next two hours, the master won $10 \%$ of the games with the remaining opponents, lost two games, and drew the remaining 7 games. On how many boards was the game being played?
## Solution.

Let the game be played on $x$ boards. When in the first 2 hours $(0.1 x+8)$ games were played, in the next 2 hours: $0.1(x-0.1 x-8)+7+2$.

Then

$0.1 x+8+0.1(x-0.1 x-8)+7+2=x$,

from which $x=20$.

Answer: on 20 boards.