The echo sounder sound wave sent towards the seabed returned after 8 seconds. What is the depth of the sea in this area?

The depth of the sea is essentially the distance that sound has traveled.
The distance can be calculated from the speed formula:
S = v * t,
where v is speed, t is time.
Let’s find the distance that the sound traveled in all 8 seconds to the bottom of the sea and back (approximate speed of sound in water is 1500 m / s):
S = 1500 m / s * 8 s = 12000 (m).
Since 12,000 m is the distance from the ship to the bottom and back from the bottom to the ship, the depth of the sea in this section will be equal to half of the entire path traversed by sound:
s = S / 2;
s = 12000/2 = 6000 (m).
Answer: the sea depth is 6000 m (6 km).