@Test
public void priceForTwoDifferentBooks_is5PercentDiscounted() {

    Map<Book, Basket.Amount> books = new HashMap<Book, Basket.Amount>();
    books.put(Book.FIRST, new Basket.Amount(1));
    books.put(Book.SECOND, new Basket.Amount(1));
    PriceCalculator calculator = new PriceCalculator(books);
    assertThat(calculator.total()).isEqualTo(15.2D);
}