Best buy sells iPhones amd Samsung. Thy have 81 phones in the store. If the number of iphones is twice the number of Samsung's how many of each phone does the story carry?

54  iphones ,27 samsungs

81/3=27

27x2=54

81-54=27

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.