Instantiate and display a ClassifiedAd
There is an error in how you implement the Price property: according to the description your get for the price should return the number of words times 0.09. As it is, the Price will always return 0 as there is never a value assigned to it.

Try adding Console.WriteLine(Article.Price); inside your Main method for demonstration.

You can add this inside your class:

private const decimal pricePerWord = 0.09M;


Notice the use of decimal (including the M suffix) instead of double, which is a more appropriate type for currency/money values.

Then your get becomes:

public decimal Price
{
    get
    {
        return Words * pricePerWord;
    }
}




I would also recommend to make a constructor for ClassifiedAd that takes the number of words, since a classified ad without words would be nonsensical.

public ClassifiedAd(int words)
{
    this.words = words;
}




Also as suggested by Roman Gräf, a ToString override method would be a nice addition:

override public string ToString()
{
    return string.Format("The price of a {0} word classified ad is {1}\n", Words, Price);
}




Then your Main method can do everything by only using what your ClassifiedAd class provides:

static void Main(string[] args)
{
    ClassifiedAd ad = new ClassifiedAd(583);
    Console.WriteLine(ad.ToString());
    // The price of a 583 word classified ad is 52.47

    ad.Words = 900;
    Console.WriteLine(ad.ToString());
    // The price of a 900 word classified ad is 81.00
}