The time taken by the ultrasonic sound waves to travel from the ship to the sea-bed, and back to the ship is 4 seconds. So, the time taken by the ultrasonic sound to travel from the ship to sea-bed will be half of this time, which is `(4)/(2) =2` second. This means that the sound takes 2 seconds to travel from the ship to the bottom of the sea.
Now, Speed `= ("Distance")/("Time")`
So, `1500 = ("Distance")/(2)`
And, Distance `= 1500 xx 2m = 3000m`
Thus, the depth of this sea below the ship is 3000 metres.