4. Sasha, Lёsha, and Kolya started a 100 m race at the same time. When Sasha finished, Lёsha was ten meters behind him, and when Lёsha finished, Kolya was ten meters behind him. How far apart were Sasha and Kolya when Sasha finished? (It is assumed that all the boys run at constant, but of course, not equal speeds.)
Answer: 19 m.

Solution: Kolya's speed is 0.9 of Lesha's speed. At the moment when Sasha finished, Lesha had run 90 m, and Kolya had run $0.9 \cdot 90=81$ m. Therefore, the distance between Sasha and Kolya was 19 m.