Adding Signed Numbers
Adding negative numbers sounds so abstract. But it really isn't if you think about some real world examples.
Let's say you have $10 and you want to buy a used video game from your friend that costs $20. If you look at it on a number line, you start off with $10 (that's a positive number), then you move to the left of the number line $20. You give your friend the $10 and you still owe him $10. So you have negative $10. The expression would look like this:
$10 - $20 = -$10
This is what it would look like on a number line:
So let's take example another step further, what if you want to buy another used video game from your friend that costs $15. You already owe your friend $10, now you're going to owe him even more. Let's take a look at the expression:
-$10 + (-$15) = what you owe your friend
What the heck? That looks a little scary. Not if you think about it. You owe your friend $10, if you buy something else from him for $15, then you owe him even more. You owe him $25. Let's take a look at it on a number line. You start at -$10 (that's how much you owe) and then you move to the left another $15.
Note 1: -25 is smaller than -$10. You have even less money when you owe $25 compared with owing $10.
Note 2: Adding a negative number is the same as subtracting a positive number. These two expressions are the same:
-$10 + (-$15) = -$10 - $15
Shows how adding a negative number is the same as subtracting a positive number.
Positive and Negative Numbers - Description of how to add and subtract positive and negative numbers including an online exercise with answers.