I've got a spreadsheet, where I'm trying to calculate the amount of retained users over time for a subscription based service. https://docs.google.com/spreadsheet/ccc?key=0ArE8l42n3soadGJGRmtZMWdpeFZKSTQycEtKRXE5ZFE#gid=0
I can get the totals but it's currently a manual process and each new week requires more manual input.
Trying to see if there is a mathematical way in Excel to figure out line B23 without having to manually build out B18 to B22. I showed this to a few other people and they said I'd have to write a small program to get the answer, but it seems like it should be possible in Excel.