The river 500 m wide is flowing with a current of 5 km/h. A boat starts from one bank of the river and wishes to cross the river at right angle to stream direction. Boatman can row the boat at 13 km/h. The time he takes to cross the river is (2) 2.5 min (4) 4.5 min (1) 1.5 min (3) 3.5 min