Question

A boat moves with a speed of 5 km/h relative to water in a river flowing with a speed of 3 km/h and having a width of 1 km. The minimum time taken around a round trip is

• A. 5 min
• B. 60 min
• C. 20 min
• D. 30 min

Answer 1

Answer: (D)

30 min

Answer 2

For the round trip he should cross perpendicular to the river $\therefore$Time for trip to that side $= \frac{1km}{4km/hr} = 0.25hr$

To come back, again he take 0.25 hr to cross the river.

Total time is 30 min, he goes to the other bank and come back at the same point.