static void Main(string[] args)
{
GradeBook book = new GradeBook();
book.AddGrade(90.5);
book.AddGrade(85.5);
book.AddGrade(75.5);
GradeStatistics stats = book.ComputeStatistics();
Console.WriteLine("List of Grades in the Grade Book");
book.DisplayGradeBook();
Console.WriteLine("Average Grade " + stats.AverageGrade);
Console.WriteLine("Highest Grade " + stats.HighestGrade);
Console.WriteLine("Lowest Grade " + stats.LowestGrade);
}