1. In the village of Big Vasyuki, there is a riverside promenade 50 meters long, running along the river. A boat 10 meters long passes by it in 5 seconds when traveling downstream and in 4 seconds when traveling upstream. How many seconds will it take for a paper boat to float from one end of the promenade to the other?
Answer: $33 \frac{1}{3}$ sec.

Solution: The speed of the boat downstream is $15 \mathrm{~m} / \mathrm{c}$, and upstream it is $12 \mathrm{~m} /$ s, so the speed of the current is $1.5 \mathrm{~m} / \mathrm{c}$. Therefore, the boat will pass by her in 50/1.5 $=33 \frac{1}{3}$ sec.

Comment for graders: The times for traveling with and against the current were swapped in the problem statement.