question_answer

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 time taken around a round trip is :

A)  5 min                                   

B)  60 min

C)  20 min                                 

D)  30 min

Correct Answer: D

Solution :

                 Time taken by the boat to reach the other side is \[Time=\frac{distance}{relative\text{ }speed}\] Relative speed\[=\sqrt{{{5}^{2}}-{{3}^{2}}}=\sqrt{16}=4\,km/h\] \[\therefore \]  \[T=\frac{1\,km}{4\,km/h}=0.25\,h\] To come back boat takes the same time, hence total time is \[T=2\times 0.25=0.5\text{ }h\] Also,         1 h = 60 min 0.5 h = 30 min