A motorboat can row at the speed of 10 km/h in still water. If the river is flowing at 5 km/h, and it takes 20 hours for a round trip, then the distance between the two places is
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → 75 km
Downstream speed = 10 + 5 = 15 km/h
Upstream speed = 10 − 5 = 5 km/h
Let the distance be d km. The total time for the round trip is 20 hours:
\( \displaystyle \frac{d}{15}+\frac{d}{5}=20 \)
\( \displaystyle \frac{d+3d}{15}=20 \)
\( \displaystyle 4d=300 \)
\( \displaystyle d=75 \)