a) 16 km
b) 18 km
c) 21 km
d) 25 km
Answer: c
Explanation: Let the distance of the destination from the starting point = x km.
Speed downstream = (10 + 4) = 14 kmph
Speed upstream = (10 – 4) = 6 kmph
Total time taken = 5 hours
$$\eqalign{
& {\frac{x}{{14}}} + {\frac{x}{6}} = 5 \cr
& \frac{{ {3x + 7x} }}{{42}} = 5 \cr
& {\text{or}},\,10x = 42 \times 5 \cr
& {\text{or}},\,x = \frac{{42 \times 5}}{{10}} \cr
& {\text{So}},\,x = 21\,{\text{km}} \cr} $$
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