Task 1. Dad and Mom are cooking pancakes on the stove, while their children Petya and Vasya are eating them. Dad can cook 70 pancakes in an hour, and Mom can cook 100 pancakes. Petya, if he tries hard, can eat 10 pancakes in 15 minutes, and Vasya can eat twice as much. After how much time will there be no less than 20 uneaten pancakes left on the table?
Answer: 24 min

Solution. Let the baking rate of pancakes be $p_{1}=\frac{170}{60}$ pancakes/min, and the eating rate of pancakes be $p_{2}=\frac{10+20}{15}=2$ pancakes/min.

In $k$ minutes, there will be no less than $q=k\left(p_{1}-p_{2}\right)$ pancakes left on the table: $k \cdot \frac{5}{6} \geq 20 \rightarrow k \geq 24$.