A boat starting from point ‘P’ on one bank of river with a sound siren of frequency f moves in the river in such a way that it crosses the river in minimum time. A detector is fixed at some height from the ground on the other bank just opposite to point P. Velocity of river flow is V, velocity of boat is 2V w.r.t. water flow and velocity of sound in air = $V_0$ . Consider only sound pulses reaching the detector through air. Then [Width of river = d and $V<<V_0$ ]