OpenDoor3.cs

// code that's testable

public class Checkout
{
    private IPriceList drinksMenu;
    private IClock clock;

    public Checkout(IPriceList drinksMenu, IClock clock)
    {
        this.drinksMenu = drinksMenu;
        this.clock = clock;
    }

    public decimal CalculateCost(List<Order> orders)
    {
        var total = 0.0m;

        // total the orders
        foreach(var order in orders)
        {
            total += this.drinksMenu.LookupPrice(order.Drink) * order.Quantity;
        }

        // half price on Friday 5pm-6pm
        var dateTimeNow = this.clock.GetDateTimeNow();
        if (dateTimeNow.DayOfWeek == DayOfWeek.Friday && dateTimeNow.Hour == 17)
        {
            total /= 2;
        }

        // calculate the tax
        total += total * 0.1m;

        return total;
    }
}


public class Order
{
    public string Drink { get; set; }
    public int Quantity { get; set; }
}


public interface IPriceList
{
    decimal LookupPrice(string itemName);
}


public interface IClock
{
    DateTime GetDateTimeNow();
}