using System;

using System.Linq;

public class Program

{

    public static void Main()

    {

        int totalGrade = 0;

        var num = new Random();

        (int score, string letter)[] gradeTable = { (90, "A"), (80, "B"), (70, "C"), (60, "D"), (0, "F") };

        for(int test = 1; test < 5; test++)

        {

            int grade = num.Next(0,100);

            var letter = gradeTable.First(test => grade>=test.score).letter;

            Console.WriteLine($"Exam {test} is {grade},{letter}");

            totalGrade += grade;

​

        }

        var avg = totalGrade / 4m;

        Console.WriteLine($"Average: {avg}");

    }

}