Question

What is positive 4 plus negative 5

Answer 1

Your answer is going to be -1. 
4+(-5) changes to 4-5, which equals -1.

Answer 2

That's the same as starting from the lobby, taking the elevator UP four floors,
then taking another elevator DOWN 5 floors.   Where do you end up ?

               (positive 4) + (negative 5)  =  -1