The speed of a boat along the stream is 12 km/hr and against the stream is 8 km/hr. The time taken by the boat to sail 24 km in still water is?

Answer: Option C

Speed of downstream
D = 12 km/h
Speed of upstream
U = 8 km/h
Speed of boat in still water
$$\begin{array}{rl}& =\frac{D+U}{2}\\ & =\frac{20}{2}\\ & =10km/h\end{array}$$
Time taken by the boat in still water
$$\begin{array}{rl}& =\frac{24km}{10km/hr}\\ & =2.4\text{hours}\end{array}$$