A man can row at 30 km/hr in still water. If the rate of current is 5 km/hr, find the average speed of man if he rows a certain distance upstream and returns back to starting point.20 km/hr.25 km/hr.32 km/hr.29.2 km/hr.

The speed of the man in still water is given as 30 km/hr. The speed of the current is given as 5 km/hr.

When the man is rowing upstream (against the current), his effective speed is his rowing speed minus the speed of the current. So, his upstream speed is 30 km/hr - 5 km/hr = 25 km/hr.

When the man is rowing downstream (with the current), his effective speed is his rowing speed plus the speed of the current. So, his downstream speed is 30 km/hr + 5 km/hr = 35 km/hr.

The average speed is given by the total distance travelled divided by the total time taken. In this case, the man travels the same distance upstream and downstream, so the total distance travelled is 2D (where D is the distance travelled in one direction).

The time taken to travel upstream is D/25 and the time taken to travel downstream is D/35. So, the total time taken is D/25 + D/35.

Therefore, the average speed is (2D) / (D/25 + D/35).

Solving this equation gives the average speed as 29.2 km/hr. So, the correct answer is 29.2 km/hr.