public void PurchaseBook_FourDifferentBooks_CorrectTotalPriceWithTwentyPercentDiscount()
        {
            ShoppingCart shoppingCart = new ShoppingCart();

            shoppingCart.AddBook1();
            shoppingCart.AddBook2();
            shoppingCart.AddBook3();
            shoppingCart.AddBook4();

            Assert.IsTrue(shoppingCart.TotalPrice == 25.6);
        }