A motorboat, whose speed is 15 km/h in still water, goes 30 km downstream and comes back in a total of 4 hours 30 minutes. What is the speed of the stream (in km/h)?
A
2 km/h
B
3 km/h
C
4 km/h
D
5 km/h
Correct Option:
D
Explanation
With stream speed x, downstream speed is 15+x and upstream speed is 15-x. Testing x=5: downstream time = 30/20=1.5 h and upstream time = 30/10=3 h. Total = 4.5 hours, exactly as required.
