To try and encourage more sales of the 5 different Harry
Potter books they sell, a bookshop has decided to offer
discounts of multiple-book purchases.
One copy of any of the five books costs 8 EUR.
If, however, you buy two different books, you get a 5%
discount on those two books.
If you buy 3 different books, you get a 10% discount.
If you buy 4 different books, you get a 20% discount.
If you go the whole hog, and buy all 5, you get a huge 25%
discount.
Note that if you buy, say, four books, of which 3 are
different titles, you get a 10% discount on the 3 that
form part of a set, but the fourth book still costs 8 EUR.
Your mission is to write a piece of code to calculate the
price of any conceivable shopping basket (containing only
Harry Potter books), giving as big a discount as possible.