A motorboat, whose speed in 15 km/hr. in still water goes 30 km downstream and comes back in a total of 4 hours 30 minutes. The speed of the stream (in km/hr.) is:
2 kmph
3 kmph
4 kmph
5 kmph
Explanation:
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.