Explain how the same object can weigh more by the ocean than it does at the
top of a mountain?

As you travel away from the Earth's surface, your mass stays the same but your weight reduces as gravitational pull decreases. ... But generally speaking, gravitational pull, and therefore weight, is lower on a mountain than at sea level

Explanation:

if the person is shown to have +90n, then ans is -10n.

if the person is shown to have -90n, then ans is +10n.