(c) 6 hrs 50 min
Let the speed of the motorboat be 36x km/hr and that of the current of water be 5x km/hr.
Speed downstream = (36x + 5x) km/hr = 41x km/hr
Speed upstream = (36x – 5x) km/hr = 31x km/hr
Time taken to travel downstream = 5 hrs 10 min
= \(5\frac{10}{60}\) hrs = \(\frac{31}{6}\) hrs
∴ Distance covered downstream = \(\big(41x\times\frac{31}6\big)\) km
⇒ Distance covered upstream = \(\big(41x\times\frac{31}6\big)\) km
∴ Time taken to travel upstream = \(\big(41x\times\frac{31}6\times\frac1{31x}\big)\) hrs
= \(\frac{41}{6}\) hrs = \(6\frac56\) hrs = 6 hrs 50 min.