# A motor-boat whose speed is 9 km/hr in still water, goes 12 km downstream

A motor-boat whose speed is 9 km/hr in still water, goes 12 km downstream and comes back in a total time of 3 hours. Find the speed of the stream.

(a) 3 km/hr

(b) 4 km/hr

(c) 6 km/hr

(d) 9 km/hr