static void Main(string[] args)
        {
            //instantiate class Gradebook => invokes constructor (defaults but can be customized)
            Gradebook book = new Gradebook();

            book.AddGrade(91);
            book.AddGrade(89);
            book.AddGrade(72);

            //separates blueprint class from computational class
            GradeStatistics stats = book.ComputeStatistics();


            Console.WriteLine(stats.AverageGrade);
            Console.WriteLine(stats.HighestGrade);
            Console.WriteLine(stats.LowestGrade);
        }
        //Gradestatistics is what will be returned - void is used if not returning a value
        public GradeStatistics ComputeStatistics()
        {
            GradeStatistics stats = new GradeStatistics();

            float sum = 0;

            foreach (float grade in grades)
            {
                stats.HighestGrade = Math.Max(grade, stats.HighestGrade);
                stats.LowestGrade  = Math.Min(grade, stats.LowestGrade);
                //adds value of grade into variable
                sum += grade;
            }

            stats.AverageGrade = sum / grades.Count;
            return(stats);
        }