13.175. A motorboat left the pier simultaneously with a raft and traveled downstream $40 / 3$ km. Without stopping, it turned around and went upstream. After traveling $28 / 3$ km, it met the raft. If the speed of the river current is 4 km/h, what is the motorboat's own speed
Solution.

The boat and the raft were on the way $\frac{\frac{40}{3}-\frac{28}{3}}{4}=1$ hour. Let $v$ km $/ h$ be the boat's own speed. Downstream, the boat traveled $\frac{\frac{40}{3}}{4+v}$ hours, and upstream $\frac{28}{3(v-4)}$ hours. According to the condition, $\frac{40}{3(v+4)}+\frac{28}{3(v-4)}=1$, from which $v=\frac{68}{3}$ km $/$ h.

Answer: $68 / 3$ km $/$ h.