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

            shoppingCart.AddBook1();
            shoppingCart.AddBook2();
            shoppingCart.AddBook3();
            shoppingCart.AddBook4();
            shoppingCart.AddBook5();

            Assert.IsTrue(shoppingCart.TotalPrice == 30);
        }