4. Vanya and Petya decided to mow the lawn for a football field. Vanya could do it alone in 5 hours, and Petya in 6 hours. They started at 11 o'clock, and stopped mowing simultaneously when their parents called them, but Petya took a one-hour lunch break, and Vanya's lunch break lasted two hours. One tenth of the lawn remained unmowed. At what time did the parents call the boys?
Answer: At 15 o'clock.

Solution: Let $t$ be the time from 11 o'clock to the end, then Petya worked $(t-1)$ hours, and Vanya worked $(t-2)$ hours. In one hour, Petya completed $\frac{1}{6}$ of the work, and Vanya completed $\frac{1}{5}$ of the work. We get the equation: $\frac{1}{6} \cdot(t-1)+\frac{1}{5} \cdot (t-2)=\frac{9}{10}$. From this, $t=4$, the time of completion is $11+4=15$.

Comment: The correct solution and the correct answer are given - 7 points. The equation is correctly set up, but due to an arithmetic error, an incorrect answer is obtained - 5 points. The problem is not solved, but there are elements of correct reasoning - 2 points. Only the correct answer and a check that it satisfies the problem's condition are provided - 3 points. Only the answer is provided - 0 points.