The total distance covered by the sound wave is twice the distance between the camera and the subject (because the wave has to reach the subject and then travel back to the camera), so 2L, where L=3.42 m. The speed of sound is v=343 m/s. It is a uniform linear motion, so we can use the basic relationship between space (S), time (t) and velocity (v) to find the time the wave needs to return to the camera:
[tex]t= \frac{S}{v}= \frac{2L}{v}= \frac{2\cdot 3.42 m}{343 m/s}=0.020 s [/tex]
