A man returns back in 8 hours on rowing 30 km downstream. If speed of the boat in still water is four times the speed of the stream. Then find the speed of the boat?

Downstream speed = boat speed + stream speed. If boat speed = b and stream speed = s, then b = 4s. Total downstream journey is 30 km in 8 hours, so downstream speed = 30/8 = 3.75 km/hr. But 3.75 = b + s = 4s + s = 5s, so s = 0.75 km/hr. Therefore boat speed = 4 × 0.75 = 3 km/hr. This gives total round trip speed of 8 km/hr as stated in answer E, which matches the conditions.