A ship is lost in a dense fog in a Norwegian fjord that is 1.80 km wide.

Question:

A ship is lost in a dense fog in a Norwegian fjord that is 1.80 km wide. The air temperature is 5.0°C. The captain fires a pistol and hears the first echo after 4.0 s.
(a) How far from one side of the fjord is the ship?
(b) How long after the first echo does the captain hear the second echo?