The altitude of a submarine is - 60 meters. If this 25 meters less than its previous altitude, what was its previous altitude?

This question is basically asking what is - 60 plus 25. It said that - 60 is less than its previous altitude which means that we need to add to get the previous altitude. When you add a negative to a positive number all you do is count backwards just like you were subtracting. - 60 + 25 equals - 35. We can check this by subtracting 25 and - 35 to get - 60.

The submarine previous altitude is - 35.