a river 4 miles wide is following at rate of 2 miles/hr . The minimum time taken by a boat to cross the river with a speed v = 4 miles / hour (in still water ) is approximately

For minimum time the velocity in direction of motion shoud be maximum. And since speed of river is perpendicular hence have no influence on time taken to cross it. Now for maximum speed boat should move perpendicular to flow of river hence tme = distance ÷ speed = 4÷4=1hr please do approve my answer