In a bookshop, I'm monitoring the daily sales of a best selling title The
bookshop is open 7 days a week. Some days are quieter than others (for no
discernable reason).

I need to know how many copies a best selling title to reorder. If sales are
increasing I will need to replace all that I've sold and order a larger
quantity than I ordered last time, if they are roughly static I'll order the
same quantity I ordered last time. If they are declining I will order fewer
than I ordered last time.

Can Excel help here?