A boat takes `2 hours` to travel `8 km` and back in still water lake.With water velocity of `4 kmph`,the time taken for going upstream of `8km` and co

Monjur Alahi

A boat takes `2 hours` to travel `8 km` and back in still water lake.With water velocity of `4 kmph`,the time taken for going upstream of `8km` and coming back is
A. 140 min
B. 150 min
C. 160 min
D. 170 min


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Correct Answer - C
`(8)/(v_(mr))+(8)/(v_(mr))=2`
`rArr v_(mr)=8 km//hr`.
`rArrt=(8)/(v_(mr)+v_(r))+(8)/(v_(mr)-v_(r))`
`rArr t=8(v_(mr)-v_(r)+v_(mr)+v_(r))/(v_(mr)^(2)-v_(r)^(2))=(16v_(mr))/(v_(mr)^(2)-v_(r)^(2))`
`=(16xx8)/(64-16)=(16xx8)/(48) hr=(8)/(3)xx60=160` min.

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