Stupid Math Tricks

I have fifteen minutes left to my lunch break, and my mind is prodding at me to Do Something with it. I have assorted things that I could be doing, but I'm not too keen on doing any of them.

One of my coworkers sent me a "stupid math trick" email. You know the kind -- pick a random number, follow these steps, and be amazed that we can predict your results! My coworker wanted to know "How do they do that?"

Usually, these are obvious ones, where somewhere along the line you both divide and multiply by your number or somesuch, and it's easy to see why the result is always the same. This one was a little weirder. The word version went like this:

1. Grab a calculator. (you won't be able to do this one in your head) [And if you can, you're not going to be impressed by the results anyway.]
2. Key in the first three digits of your phone number (NOT the area code)
3. Multiply by 80
4. Add 1
5. Multiply by 250
6. Add the last 4 digits of your phone number
7. Add the last 4 digits of your phone number again.
8. Subtract 250
9. Divide number by 2
Do you recognize the answer?

I translated the word version of this problem into the following formula:
( Steps to the solution, for no good reason )