public void when_buy_three_different_series_books_then_amount_should_be_270()
 {
     BooksAmountShouldBe(
         books: new List <Book>
     {
         PotterBooks.First(1),
         PotterBooks.Second(1),
         PotterBooks.Third(1),
     },
         expected: 270m);
 }
 public void when_buy_five_different_series_books_then_amount_should_be_375()
 {
     BooksAmountShouldBe(
         books: new List <Book>
     {
         PotterBooks.First(1),
         PotterBooks.Second(1),
         PotterBooks.Third(1),
         PotterBooks.Fourth(1),
         PotterBooks.Fifth(1),
     },
         expected: 375m);
 }