A) 2h
B) 2 h 40 min
C) 1 h 20 min
D) Cannot be estimated from the information given
Correct Answer: B
Solution :
Boat covers distance of 16 km in a still water in 2h ie, \[{{v}_{B}}=\frac{16}{2}=8\text{km/h}\] Now, velocity of water,\[{{v}_{w}}=4\text{km/h}\] Time taken for going upstream \[{{t}_{1}}=\frac{8}{{{v}_{B}}-{{v}_{w}}}=\frac{8}{8-4}=2h\] (As water current oppose the motion of boat) time taken for going downstream \[{{t}_{2}}=\frac{8}{{{v}_{B}}+{{v}_{w}}}=\frac{8}{8+4}=2h\] (As the water current helps the motion of boat) \[\therefore \] Total time \[={{t}_{1}}+{{t}_{2}}=\left[ 2+\frac{8}{12} \right]h\] or 2h 40 min
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