Recibimos por radio un mensaje del ferry que dice: "estamos a 70km de santa cruz y vamos hacia allí a 60km/h? a) ¿a que distancia de santa cruz estará el ferry dentro de cuarenta minutos?

a)    t = 0.75 s,  b)  t = 2.25 s

Explanation:

The speed of sound is constant in a material medium

           v = 340 m / s

we can use the relations of uniform motion to find the time

          v = x / t

          t = x / v

In the exercise, the observer's distance to the wall is indicated d = 510 m, it also indicates that the shot is fired at the midpoint

          x = d / 2

a) direct sound the distance from the observer to the screen is

           x = 510/2 = 255 m

           t = 255/340

           t = 0.75 s

b) echo sound.

In this case the sound reaches the wall bounces, the distance is

             x = d / 2 + d

             x = 3/2 d

             x = 3/2 510

             x = 765 m

the time is

              t = x / v

              t = 765/340

              t = 2.25 s