Question

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

Answer 1

For the round trip he should cross perpendicular to the river.

Time for trip to that side:

$$t=\frac{1}{5}=0.2\ hr=12\ min$$

To come back, again he takes 0.2 hr to cross the river.

The minimum time taken around a round trip is

$$T=2t=24\ min$$