public enum Grade
    {
        Failing,        // = 0
        BelowAverage,   // = 1
        Average,        // = 2
        VeryGood,       // = 3
        Excellent       // = 4
    }

static void Main(string[] args)
{
    Grade grade = Grade.Average;
    Console.WriteLine(grade.ToString());    // Print Avarage
    Console.WriteLine(grade.ToString("D")); // Print 2
    Console.WriteLine(grade.ToString("X")); // Print 00000002
    Console.WriteLine((int) grade);         //Print 2
    Console.ReadKey();
}

public enum Grade
{
    Failing = 5,
    BelowAverage = 10,
    Average = BelowAverage + 5, // = 15
    VeryGood = 18,
    Excellent = 20
}

public enum Grade : byte
{
    Failing = 5,
    BelowAverage = 10,
    Average = BelowAverage + 5, // = 15
    VeryGood = 18,
    Excellent = 20
}

Console.WriteLine(grade.ToString("X")); // Print 0F

Console.WriteLine("Underlying type: {0}", Enum.GetUnderlyingType(grade.GetType())); // Print System.Byte

Console.WriteLine("Type Code      : {0}", grade.GetTypeCode()); // Print Byte

Console.WriteLine("Value : {0}", (int)grade); // Print 15