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1. 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:

A. 16 km

B. 18 km

C. 21 km

D. 25 km

Solution:

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{aligned} \frac{x}{14} + \frac{x}{6} = 5 \\ \frac{3 x + 7 x}{42} = 5 \\ or, 10 x = 42 \times 5 \\ or, x = \frac{42 \times 5}{10} \\ So, x = 21 km \end{aligned}

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