A man crosses a 320 m wide river perpendicular to the current in 4 minutes. If in still water he can swim with a speed 5/3 times that of the current, then the speed of the current, in m/min is

Resultant velocity of man w.r.t. river, vR=3204=80m/min . Let v  be the velocity of river water, the velocity of man =53v ∴    53v=vR2−v2  or  259v2=802−v2 or   169v2=802   or  43v=80 or      v=80×34=60m/min