A swimmer swims from a point P against the current for 6 min and then swims back along the current for next 6 min and reaches at a point Q. If the distance between P and Q is 120 m then the speed of the current (in km/h ) is:

Let swimmer's speed in still water be v m/s and current speed be s m/s. Against current for 6 min (360 sec): distance = (v - s) × 360. With current for 6 min (360 sec): distance = (v + s) × 360. Net displacement between P and Q = distance with current - distance against current = [(v + s) - (v - s)] × 360 = 2s × 360 = 720s meters. Given net displacement is 120 m, so 720s = 120, s = 120/720 = 1/6 m/s = (1/6) × (3600/1000) km/h = 0.6 km/h.