From the given question, the surface of the water body is the reference point. And this point is assumed to be 0, so that any distance above it is positive, and any distance below is negative. This is synonymous to the number line system.
Thus,
For the aircraft, its height = +200 m
For the submarine, its depth = -55 m
So that the difference between the submarine and aircraft can be determined as;
200 - (-55)
= 200 + 55
= 255 m
The distance between the submarine and aircraft is 255 m.
