Problem 3.1. Pasha knows the speed of his motorboat. He calculated that it would take him 20 minutes to travel from the pier to the bridge and back. But in his calculations, he forgot that the river has a current. How many minutes will Pasha actually need for the planned route, if it is known that the speed of the current is exactly 3 times less than the speed of the motorboat? (The speeds of the motorboat and the current are constant.)
Answer: 22.5.

Solution. Let $x$ be the speed of the current, then $3x$ is the speed of the boat. According to Pasha's calculations, traveling at a speed of $3x$, he would spend 10 minutes on the trip in one direction.

To cover the same distance downstream, traveling at a speed of $4x$ (which is $\frac{4}{3}$ times the speed of the boat in reality), he would spend $\frac{3}{4} \cdot 10 = 7.5$ minutes.

And to cover the same distance upstream, traveling at a speed of $2x$ (which is $\frac{3}{2}$ times less than the speed of the boat in reality), he would spend $\frac{3}{2} \cdot 10 = 15$ minutes.

Therefore, the entire route will take Pasha $7.5 + 15 = 22.5$ minutes.