The velocity of a motor boat with respect to still water is 7 ms−1  and velocity of water in a river is 3 ms−1 . When the boat starts moving upstream a float was dropped from it. The boat travels 4.2 km and returns and reaches the float. The time taken by the boat to catch the float is

When the boat is in upstream. Relative velocity between boat and float is = 10 m/sec The separation between them when boat moves 4.2 km is S=Vrelt VreldVboat=10×4.2×10007 = 6000 mWhen the boat is in down stream, relative vrel=4m/sec  time taken to cover the separation distance 6000m is t1=60004=1500sec=25min Total time from starting of journey isTe=25+dVboat =25+42007×60= = 35 min