The intensity of sound is just like the force of gravity, the force between electric charges, and the intensity of light . . . they all DEcrease at the same rate that the SQUARE of the distance INcreases.
So if two people are watching or listening to the same source, and one intensity is 1/10 as intense as the other intensity, then the farther person must be √10 times as far from the source as the nearer person is.
√10 = 3.1622 ...
So the second guy is about 3.16 miles from the fire truck.