A motorboat whose speed is 15 km/hr in still water goes 30km downstream and comes back in four and a half hours. The speed of the stream is ?

Let the speed of the stream be x km/hr. Then,
Speed downstream = (15 + x) km/hr
Speed upstream = (15 – x) km/hr

We know from question that it took 4(1/2)hrs to travel back to same point.
So, (30/15 + x) + (30/15 − x) = 4(1/2)
=> 900/225 − x² = 9/2
=> 9x² = 225
=> x = 5km/hr.