Q. If a boat goes 100 km downstream in 10 hours and 75 km upstream in 15 hours, then the speed of the stream is:
Answer: 2.5 km/hr
Notes: Rate downstream = 10 kmph Rate upstream = 5 kmph ∴ Speed of current = $ \frac{1}{2}\left ( 10-5 \right )kmph$ $ =>2.5km/hr$ Hence option [D] is the right answer.