How do you run a report or formula for the following principle example?

Based on a sales competition, I have 100 days to get my daily average income
to a total average of $45. I am averaging $30 income daily through the first
61 days. How do I run a report that can tell me (instead of manually doing
it) the amount of daily income I need to average for the remaining 38 days in
order to meet my goal of $45.00 for the 100-day period? I am using 2003
version.

thanks....

A.W.