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, $v_R=\frac{320}{4}=80m/\min$ . Let $v$  be the velocity of river water, the velocity of man $=\frac{5}{3}v$

$\therefore$    $\frac{5}{3}v=\sqrt{v_R^2-v^2}$  or  $\frac{25}{9}{v}^2={80}^2-v^2$

or   $\frac{16}{9}{v}^2={80}^2$   or  $\frac{4}{3}v=80$

or      $v=\frac{80\times 3}{4}=60m/\min$