it would be 4000 meters, since it took 5 seconds and it travels at a rate of 1600 meters per second (1600 meters for every second it travels), you would multiply 1600 by 5 (for the 5 seconds) equaling 8000, which is total distance traveled (from ship to seabed and back to ship). you would then Divide that by two to get the distance between the ship and the seabed (just ship to seabed). or you can put it in one equation and get this (1600*5)/2=D (D=distance between the ship and the seabed.
