The speed of a boat downstream is 15 km/hr and the speed of current is 3 km/hr. Find the total time taken by the boat to cover 15 km upstream and 15 km downstream.

Answer: Option A

Given,
Speed of boat in downstream = 15 km/hr
Speed of current = 3 km/hr
Speed of boat in still water = 12 km/h
Time taken at upstream
$$\begin{array}{rl}& =\frac{15}{12-3}\\ & =\frac{15}{9}\text{hr}\\ & =1\text{hour}40\text{minutes}\end{array}$$
Time taken at downstream
$$\begin{array}{rl}& =\frac{15}{12+3}\\ & =\frac{15}{15}\text{hr}\\ & =1\text{hour}\end{array}$$
∴ Total time = 2 hours 40 minutes