Q:
A motorboat goes 8 km an hour in still water, but takes thrice as much time in going the same distance against the current than going with the current. Then find the speed of the current?
Answer & Explanation
Answer: A) 4 kmph
Explanation: Let the speed of current = 'C' km/hr
Given the speed of boat in still water = 6 kmph
Let 'd' kms be the distance it covers.
According to the given data,
Boat takes thrice as much time in going the same distance against the current than going with the current
i.e, $\frac{\mathbf{d}}{\mathbf{8}\mathbf{}\mathbf{-}\mathbf{}\mathbf{C}}\mathbf{}\mathbf{=}\mathbf{}\mathbf{3}\mathbf{}\mathbf{\times}\mathbf{}\frac{\mathbf{d}}{\mathbf{8}\mathbf{}\mathbf{+}\mathbf{}\mathbf{C}}$
$\Rightarrow 24-3\mathrm{C}=8+\mathrm{C}\phantom{\rule{0ex}{0ex}}\Rightarrow 4\mathrm{C}=16\phantom{\rule{0ex}{0ex}}\mathbf{\Rightarrow}\mathbf{}\mathbf{C}\mathbf{}\mathbf{=}\mathbf{}\mathbf{4}\mathbf{}\mathbf{kmph}$
Hence, the speed of the current C = 4 kmph.
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