Problem 1.7

Two swimmers leave point $A$ on one bank of the river to reach point $B$ lying right across on the other bank. One of them crosses the river along the straight line $A B$ while the other swims at right angles to the stream and then walks the distance that he has been carried away by the stream to get to point $B$. What was the velocity $u$ of his walking if both swimmers reached the destination simultaneously? The stream velocity $v_{0}=2.0$ km/hour and the velocity $v^{\prime}$ of each swimmer with respect to water equals $2.5 \mathrm{~km}$ per hour.