Question

A boat moves with a speed of 5km/h relative to water in a river flowing with a speed of 3km/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

The correct option is D 30 min

correct answer is D.

Time for trip on one sides

$width=1km$

$T=\frac{width}{speed}$

$=\frac{1}{\sqrt{5^2-3^2}}$

$=\frac{1}{\sqrt{16}}$

$=\frac{1}{4}$

$=0.25km/h$

total time for round trip

$T=2t$

$=2\times 0.25$

$=0.5hours$

$=30minutes$