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A boat takes two hours to travel 8 km and come back in still water lake. If the velocity of water is 4 kmph the time taken for going upstream of 8 km and coming back is

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2 hours 20 minutes

2 hours 40 minutes

1 hour 20 minutes

3 hours 30 minutes

answer is B.

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Detailed Solution

Velocity of boat $v_{b} = \frac{8 + 8}{2} = 8 kmph$

Velocity of water $v_{w} = 4 kmph$

Up stream $d = (v_{b} - v_{w}) t_{1}$

$8 = 4 \times t_{1}$

$t_{1} = 2 h$

Down stream $d = (v_{b} - v_{w}) t_{2}$

$t_{2} = \frac{2}{3} h$

$= \frac{2}{3} \times 60 \min = 40 \min$

Total time T = 2hours 40 minutes

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