A river is flowing from west to east at a speed of 5 metres per minute. A man on the south bank of the river, capable of swimming at 10 metres per minute in still water, wants to swim across the river in the shortest time. He should swim in a direction.
(a) due north
(b) 30° east of north
(c) 30° north of west
(d) 60° east of north.

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Solution

(a) due north

If the man swims at any angle east to the north direction, although his relative speed will increase, he will have to travel a larger distance. So, he will take more time.
If the man swims at any angle west to the north direction, his relative speed will decrease. So, he will take more time.

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