A man wishes to cross a river in a boat. If he crosses the river in minimum time he takes 10 minutes with a drift of 120 m. If he crosses the river taking shortest route, hetakes 12.5 minutes, find velocity of the boat with respect to water.

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20 m/min

12 m/min

10 m/min

8 m/min

answer is A.

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Detailed Solution

10=Lv ......(1)12.5=Lv2−u2=Lv1−u2/v2 ..(2)from (i) and (ii) 1012.5=Lv×v1−u2/v2L45=1−122v21625=1−122v2⇒122v2=1−1625=92512v=35⇒v=12×53=20m/min

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