question_answer

The speed of a motor launch with respect to water in a stream is 8 m/s, while water current?s speed is 3 m/s. When the launch began travelling upstream, a float was dropped from it. After travelling a distance of 4.8 km upstream, the launch turned back and caught up with the float. What is the total time which elapsed during the process?

A) 32 min             

B) 24 min

C) 16 min             

D) 8 min

Correct Answer: A

Solution :

Time taken by launch from \[A\text{ }\to \text{ }B\text{ }\to \text{ }C\] is equal to time taken by float from A to C: -----------------------                                    \[\to u=3\,\,m/s\] \[\frac{4800}{8-3}\,\,+\,\,\frac{4800+x}{8+3}\,\,=\,\frac{x}{3}\] Solve to get time taken:                 \[t=\frac{x}{3}\,\,=\,\,32\,\,\min \]