Let's call L the distance between the tugboat and the reflecting object. The sound wave travels from the tugboat to the reflecting object, and then back again to the tugboat, in a total time of t=3.20 s. This means that the distance covered during this time is S=2L.
The sound wave moves by uniform motion, with velocity v=342 m/s, so we can use the basic relationship between space and time for a uniform motion:
[tex]S=vt = (342 m/s)(3.20 s)=1094.4 m[/tex]
And so the distance L between the tugboat and the reflecting object is
[tex]L= \frac{S}{2}= \frac{1094.4 m}{2}=547.2 m [/tex]
