When you add integers, why do you subtract? -9+5=-4. I do not understand why there is an addition sign. How do you know when you are suppose to subtract.

Answer 1

Let's suppose you owe someone $9.00 (-9). You pay him $5.00 (+5). You'll still owe him $4.00 (-4).

This integer line may also help you visualize this problem.

http://education.smarttech.com/NR/rdonlyres/98A932F3-0E77-4C02-ADD0-04D9975ACDEA/0/NumberlineIntegers.gif