I'm not exactly sure where this question fits so I decided to post it here after trying several forums with no luck in receiving any responses. Hopefully someone here will be able to help me out since so many other forums have fallen short of my expectations.

I'm taking a mandatory math class for my arts degree and have never been great, or good, at math. I'm working on my assignment that is due on Tuesday and have come to a stall with this question:

Find the sum of:

(1000-200) + (995-198) + (990-196) + ... + (505-2)

To solve it I am using these formulas:

n= An-A1/d+1

sum=A1+An/d *n