A swimmer, capable of swimming at a speed of 1.60 m/s in still water (i. e., the swimmer can swim with a speed of 1.60 m/s relative to the water), starts to swim directly across a 1.25-km-wide river. However, the current is 0.549 m/s, and it carries the swimmer downstream. (a) How long does it take the swimmer to cross the river

  t = 781.25 s

Explanation:

This is an exercise in velocity composition, if we set a reference system where the x-axis is perpendicular to the river and the y-axis is parallel to the river.

The swimmer has a velocity on the x axis

           vx = 1.60 m / s

a velocity on the y axis, created by the current of the river

           vy = 0.549 m / s

time is a scalar, therefore the time it takes to cross the river is the same time it creates the displacement in e; Axis y

X axis

            vₓ = x / t

            t = x / vₓ

            t = 1250 / 1.6

            t = 781.25 s

in this time a distance has descended

            y = v_y t

            y = 0.549 781.25

            y = 428.9 m