The essence of the question is that your velocity relative to the ground is the sum of the velocities of the boat and the river.Originally Posted by bobby77
You row at right angles. Hence your velocity across the river=4km/h
Time taken=.018km/(4km/h) = .0045h=16.2sec.
In this case you row at such an angle that the resultant of velocity along the river is zero.
Let you row at an angle with the land.
Velocity acros the river is .
Hence time taken =