The time taken by ultrasonic sound to reach a SONAR receiver is 3 seconds. What is the depth of the sea in this region? (Speed of sound in water = 1500 m/s).

Given,
Time, t = 3s
Speed of sound in water given in the problem, v_w = 1500 m/s
It is known that the distance between the transmission and receiver point is 2d.
Therefore, 
$2\mathrm{d}=\mathrm{v}\times \mathrm{t}$ 

On substituting the values, we get
$\Rightarrow 2\mathrm{d}=1500\times 3$ 

$\Rightarrow \mathrm{d}=\frac{4500}{2}$

 $\Rightarrow \mathrm{d}=2250 \mathrm{m}$

So, the depth of the sea is 2250m.