How would you do the problem with pencil, paper, and a calculator? Probably something like
Answer = length width (1 + overage/100)
That mortgage example was close to the first thing I ever did, being near the front of a primer I read a few years ago, so it can't be that hard to grasp. I've since added an option for number of months, a new PI popup window, and a "Last Modified" script, but that was originally copied verbatim from the book.
BTW, does this problem have a form or does it get its data from prompts?