Adding Signed Numbers
Adding negative numbers sounds so abstract. But it really isn't if you think about some real world examples.
Example 1
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:

Example 2
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

Video Tutorials


Shows how adding a negative number is the same as subtracting a positive number.

Other Tutorials
Positive and Negative Numbers  Description of how to add and subtract positive and negative numbers including an online exercise with answers.