Question

the altitude of a plane is 1600ft. what must be the altitude of a submarine so that the sum of their altitudes is 0.

Answer 1

The submarine must be -1600 ft

Answer 2

Two numbers that add up to zero are called additive inverses or opposites.
You are looking for the additive inverse of 1600 ft.
It is -1600 ft.

Answer: -1600 ft