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.
