A man goes downstream with a boat to some destination and returns upstream to his original place in 5 hours. If the speed of the boat in still water and the stream are 10 kmph and 4 kmph respectively, the distance of the destination from the starting place is:

Answer: Option C

Let the distance of the destination from the starting point = x km.
Speed downstream = (10 + 4) = 14 kmph
Speed upstream = (10 - 4) = 6 kmph
According to the question,
Total time taken = 5 hours
$$\begin{array}{rl}& \frac{x}{14}+\frac{x}{6}=5\\ & \frac{3x+7x}{42}=5\\ & \text{or},10x=42\times 5\\ & \text{or},x=\frac{42\times 5}{10}\\ & \text{So},x=21\text{km}\end{array}$$