Here's one for those of you who have been following along with your own spreadsheet (e-mail me for your copy).
Quiz: Using the customer retention index line (cells C5 - G5), figure out how much you have to increase customer retention in order for this business to hit $80,000,000 in Year 5.
Use the comments section of this post to outline your solution. There are many different solutions that get you to the same answer.
Buy the book here, if you are so inclined!