A motorboat whose speed is 9 km/hr in still water, goes 15 km downstream and comes back in a total time of 3 hours 45 minutes. Find the speed of the stream.
Let the speed of stream be x km/h
Speed of boat is 9km/hr
Speed downstream = (9 + x)km/h
Speed upstream = (9 – x)km/h
Distance covered downstream = Distance covered upstream = 15km
Total time taken = 3 hours 45 minutes 
taking LCM
81 – x2 = 72 cross multiplying
x2 = 81 – 72
x2 = 9 taking square root
x = 3 or – 3 (rejecting negative value)
Hence the speed of stream is 3 km/hr
53