9.3. Three runners - Anton, Seryozha, and Tolya - are participating in a 100 m race. When Anton finished, Seryozha was 10 meters behind him, and when Seryozha finished, Tolya was 10 meters behind him. What distance was Tolya from Anton when Anton finished? (It is assumed that all boys run at constant, but of course, not equal speeds.)
Answer: 19 m.

Solution: Since Tolya's speed is $9 / 10$ of Seryozha's speed, by the time Anton finished, Tolya had run $9 / 10$ of the distance covered by Seryozha, which is $90 \cdot 9 / 10=81$ m. Thus, when Anton finished, Tolya and Anton were 19 m apart.