draw the velocity vectors. They form a 3-4-5 right triangle, with the straight-across side of length 3.
So, scale it up by a factor of 5/3 to get an equivalent distance triangle. He will have to row 25/3 km along his path, while drifting downstream 20/3 km, landing exactly 5 km from where he started.
a man can row a boat at 5km/hr. in still water and waits to cross a river to a position exactly opposite his starting point. If the river is 5km wide and is flowing at 4km/hr. eastwards find by scale drawing the direction in which he must set off in order to accomplish his objective. How long will it take to cross the river
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