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Question

Ritu can row downstream 20 km in 2 hours, and upstream 4 km in 2 hours. Find her speed of rowing in still water and the speed of the current.


A
Speed of rowing in still water: 10 km/hr and the speed of the current: 5 km/hr.
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B
Speed of rowing in still water: 8 km/hr and the speed of the current: 7 km/hr.
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C
Speed of rowing in still water: 18 km/hr and the speed of the current: 12 km/hr.
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D
Speed of rowing in still water: 6 km/hr and the speed of the current: 4 km/hr.
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Solution

The correct option is C Speed of rowing in still water: 6 km/hr and the speed of the current: 4 km/hr.
Let, speed of Ritu in still water be $$ x$$ km/hr
and speed of current be $$ y$$ km/hr

Then speed downstream $$= (x + y)$$ km/hr
speed upstream $$= (x - y)$$ km/hr

we know, $$Time = \dfrac{Distance}{Speed} $$

Then $$\cfrac {20}{x+y}=2$$ and $$\cfrac {4}{x-y}=2$$
$$\Rightarrow x + y = 10$$ ...(1)
&   $$x-y=2$$ ...(2)
adding both we get, $$2x=12\Rightarrow x=6$$
Thus using (1) $$y = 10-6=4$$

Hence the speed of Ritu in still water is $$6$$ km/hr and the speed of the current is $$4$$ km/hr.

Mathematics

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