static void Main(string[] args)
{
GradeBook book = new ThrowAwayGradebook();
book.AddGrade(91);
book.AddGrade(89.5f);
book.AddGrade(75);
GradeStatistics stats = book.ComputeStatistics();
WriteResult("Average", stats.AverageGrade);
WriteResult("Highest", stats.HighestGrade);
WriteResult("Lowest", stats.LowestGrade);
WriteResult("Grade", stats.LetterGrade);
}
