a) 5 hours 50 minutes
b) 6 hours 50 minutes
c) 6 hours
d) 12 hours 10 minutes
Answer: b
Explanation: Let the speed of the motor boat = 36x kmph and speed of current = 5x kmph.
The boat goes along with the current in 5 hours 10 minutes = $$\frac{{31}}{6}$$ hour
$$\eqalign{
& {\text{Hence, Distance}} \cr
& = {\frac{{31}}{6}} \times \left( {36x + 5x} \right) \cr
& = \frac{{41x + 31}}{6}\,km \cr
& {\text{Speed of boat upstream}} \cr
& = 36x + 5x = 41x\,{\text{kmph}} \cr
& {\text{Hence, time taken to come back}} \cr
& = {\frac{{ {41x \times {\frac{{31}}{6}} } }}{{31x}}} \cr
& = \frac{{41}}{6}\,{\text{hours}} \cr
& = 6\,{\text{hours}}\,\,50\,{\text{minutes}} \cr} $$
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