Question

A river flows due north with a velocity of 3kph. A man rows a boat across the river. His velocity relative to the water is 4kph due east. If the river is 1km wide, how long will it take him to cross the river? 

Answer 1

The velocity of man in northward direction does not affect its speed in eastern direction which is 4km/h. 

Thus time taken = distance/speed

$$\dfrac{1\ km}{4\ km/h}=\dfrac{1}{4} \ h=15\ min$$