A boat has speed of 5 km/h  in still water crosses a river of width 1 km  along the shortest possible path in15 min . The velocity of the river water in km/h  is

Shortest possible path comes when absolute velocity of boatman becomes perpendicular to river current as shown in figure.    t=ωvb=ωvbr2−vr2  ∴14=125−vr2Solving this equation, we get vr=3 km/h