The speed of a motor-boat is that of the current of water as 36:5. The boat goes along with the current in 5 hours 10 minutes. It will come back in:

Answer: Option C

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
$$\begin{array}{rl}& \text{Hence, Distance}\\ & =\frac{31}{6}\times \left(36x+5x\right)\\ & =\frac{41x+31}{6}km\\ & \text{Speed of boat upstream}\\ & =36x+5x=31x\text{kmph}\\ & \text{Hence, time taken to come back}\\ & =\frac{41x\times \frac{31}{6}}{31x}\\ & =\frac{41}{6}\text{hours}\\ & =6\text{hours}50\text{minutes}\end{array}$$