A community tool-sharing club has provided the following description of their tool rental system:

"Our club lets local residents borrow professional-grade DIY tools. We charge rental fees based on the category of the tool, not how many days it is kept. If a member borrows an accessory kit, like drill bits or safety goggles, alongside a main tool, we don't charge any extra. I prefer to run the administration on Sunday afternoons because I volunteer at a local animal shelter during the week. I need a software system to help me keep track of which member currently has which tool borrowed. Fortunately, every tool has a unique barcode ID, and each member has a unique membership card number, so it is very easy to register them in the system. The standard loan period is 7 days, and late returns are charged a penalty of £12 for every extra week or part-week (for example, returning a tool 3 days late is rounded up to a 1-week penalty of £12, whereas 8 days late is rounded up to a 2-week penalty of £24)."

The developer has decided that the following two points are the only important details from the client's request:

• The rental charge is based on the tool category and not how many days it is kept.
• Late returns incur a penalty of £12 for every extra week.