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Solving an Equation with Variables on Both Sides

A boat takes ...

Question

A boat takes $2$ hours to go $8 k m$ and come back in still water lake. The time taken for going $8 k m$ upstream and coming back with water velocity of $4 k m / h r$ is:

140 m i n

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150 m i n

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160 m i n

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170 m i n

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Solution

The correct option is C $160 m i n$
Velocity of boat in still water $V_{b} = \frac{8 + 8}{2} = 8 k m / h$
Upstream motion :
Velocity of boat $v_{1} = 8 - 4 = 4 k m / h$
Time taken $t_{1} = \frac{8}{4} = 2 h r = 120 m i n$
Downstream motion :
Velocity of boat $v_{2} = 8 + 4 = 12 k m / h$
Time taken $t_{2} = \frac{8}{12} = \frac{2}{3} h r = 40 m i n$
So, total time taken $t = 120 + 40 = 160 m i n$

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a boat takes 2hrs to travel 8km and back in still water lake.with water velocity of 4kmph,the time taken for going upstream of 8km and coming back is

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