Problem 4. There are 10 identical swimming pools and two hoses with different pressures. It is known that the first hose fills a pool 5 times faster than the second. Petya and Vasya each started filling 5 pools, Petya with the first hose, and Vasya with the second. It is known that Petya finished an hour earlier. How long did it take Vasya to fill his 5 pools?

Answer: 1 hour 15 minutes.
Solution. Let Petya fill one pool in time $x$, then Vasya fills one pool in time $5 x$ (since the second hose fills one pool five times slower).

Then Petya will fill his five pools in time $5 x$, and Vasya his in $25 x$.

We get the equation $25 x-5 x=1$, from which $x=3$ min. Then Vasya will fill his pools in $25 x=25 \cdot 3=75$ min $=1$ hour 15 min.

## Criteria

## 1 p. Only the correct answer without justification.

3 p. The time to fill one pool by any hose is correctly found.

4 p. The correct answer and justification are provided.