A man can swim with a speed of 4.0 km/h in still water. How long does he take to cross a river 1.0 km wide if the river flows steadily at 3.0 km/h and he makes his strokes normal to the river current? How far down the river does he go when he reaches the other bank?
Figure showing the velocities of swimmer and river
Given:
Speed of the man = Vm = 4km/hr in still water
Width of the river = d = 1km
Speed of the river = Vr = 3 km/hr
Explanation: The time taken by the man to cross the river is the ratio of width of the river and speed of the man. The velocity displaces the man down the river.
= 
= 0.25hr
= 15 min.
Distance covered due to the flow of river = l= Vr×t
= 3 km/hr × 
hr
= 3/4 km
= 750m
Time taken by man to cross the river is 15 min and distance covered due to flow of water is 750m down the river.