New Exercise book-store

exercises/book-store/description.md

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+To try and encourage more sales of the 5 different books
+they sell of a popular series, a bookshop has decided to
+offer discounts of multi-book purchases. 
+
+One copy of any of the five books costs $8. 
+
+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 buy all 5, you get a 25% discount. 
+
+Note: that if you buy 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. 
+
+Your mission is to write a piece of code to calculate the
+price of any conceivable shopping basket (containing only
+books of the same series), giving as big a discount as
+possible.
+
+For example, how much does this basket of books cost?
+
+- 2 copies of the first book
+- 2 copies of the second book
+- 2 copies of the third book
+- 1 copy of the fourth book
+- 1 copy of the fifth book
+
+One way of grouping these 8 books is:
+
+- 1 group of 5 --> 25% discount (1st,2nd,3rd,4th,5th)
+- +1 group of 3 --> 10% discount (1st,2nd,3rd)
+
+This would give a total of:
+
+- 5 books at a 25% discount
+- +3 books at a 10% discount
+
+Resulting in:
+
+- 5 x (8 2.00) == 5 x 6.00 == $30.00
+- +3 x (8 0.80) == 3 x 7.20 == $21.60
+
+For a total of $51.60
+
+However, a different way to group these 8 books is:
+
+- 1 group of 4 books --> 20% discount  (1st,2nd,3rd,4th)
+- +1 group of 4 books --> 20% discount  (1st,2nd,3rd,5th)
+
+This would give a total of:
+
+- 4 books at a 20% discount
+- +4 books at a 20% discount
+
+Resulting:
+
+- 4 x (8-1.60) == 4 x 6.40 == $25.60
+- +4 x (8-1.60) == 4 x 6.40 == $25.60
+
+For a total of $51.20
+
+And $51.20 is the price with the biggest discount.

exercises/book-store/metadata.yml

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+---
+blurb: "To try and encourage more sales of different books from a popular 5 book series, a bookshop has decided to offer discounts of multiple-book purchases."
+source: "Inspired by the harry potter kata from Cyber-Dojo."
+source_url: "http://cyber-dojo.org"