Ravish rowed 40 km downstream in 2 hrs 30 min and returned to the starting point in 5 hrs. What is the speed of the current ?

4 km/hr

Distance $=40$ km.

Downstream time $=2$ hr $30$ min $=\frac{5}{2}$ hr.

Downstream speed $=\frac{40}{\frac{5}{2}}=16$ km/hr.

Upstream time $=5$ hr.

Upstream speed $=\frac{40}{5}=8$ km/hr.

Speed of current $=\frac{16-8}{2}=4$ km/hr.

final answer: $4$ km/hr