Excel Formula- Representing Representative Sales

[cmh5286]

I have in my spreadsheet the number of reps we're using. We can guess that the reps will sell approx. 5,000 their first year, 7.5 their second, and 10 their third and max out at about 12 their fourth. I need a formula that can tell if there's a change in the previous month's number of reps and represent an estimate of what they will sell monthly over the next few years. If it wasn't for the possibility of multiple hires I could do this but I'm kind of in a jam as a result. Any help would be appreciated!