arithmetic - a motorboat whose speed in 15 kmhr in still water goes 30 km downstream and comes back in a total of 4 hours 30 minutes the speed of the stream in kmhr is - skillgun

Let the speed of the stream be x km/hr. Then,
Speed downstream = (15 + x) km/hr,
Speed upstream = (15 - x) km/hr
So we know from question that it took 4(1/2)hrs to travel back to same point.
So,30/(15+x)-30/(15-x)=41/2
=> 900/(225-x^2) = 9/2
=> 9x^2 = 225
=> x = 5 km/hr