Did you hear that?

Relevant wiki: Sound

The formula for calculation of speee of sound in air is $\dfrac{2d}{t}$ .
In first case, v = $\dfrac {2d}{4} \rightarrow \boxed{1}$

In second case, v = $\dfrac {2 (d + 125)}{5} \rightarrow \boxed{2}$

Equating $\boxed{1}$ and $\boxed{2}$ , we get:-
$\dfrac{2d}{4} = \dfrac{2(d + 125)}{5}$
$5d = 4d + 500$
$d = \boxed{500}m$

Use the distance = rate of speed X time formula.

Let w = width of river. The 2*w is the distance the sound had to travel across, bounce off cliff and come back in 4 seconds.

2 * w = r * 4 seconds, where r = speed of sound

The next situation distance = 2 * w + 2 * 125 = 2*w + 250. Speed of sound is the same. Time = 5 seconds.

2 w + 250 = r * 5. From equation 1, r = 2 w / 4 = 0.5 *w

Substituting: 2 w + 250 = 0.5 w * 5

2 * w + 250 = 2.5 * w

250 = 0.5 * w

500 = w