public void PurchaseBook_FiveDifferentBooks_CorrectTotalPriceWithTwentyFivePercentDiscount()
{
ShoppingCart shoppingCart = new ShoppingCart();
shoppingCart.AddBook1();
shoppingCart.AddBook2();
shoppingCart.AddBook3();
shoppingCart.AddBook4();
shoppingCart.AddBook5();
Assert.IsTrue(shoppingCart.TotalPrice == 30);
}