How to do this C# question?? Please someone help me..

Try this:

namespace ConsoleApplication1
{

    class Student
    {
        public string sName { get; set; }
        public int iMarks { get; set; }
        public string sGrade { get; set; }

        public Student(string name, int marks)
        {
            sName = name;
            iMarks = marks;
        }
    }

    class Program
    {
        public static void Main(string[] args)
        {
            try
            {
                int N = 0;
                decimal Avg = 0;
                decimal Min = 9999;
                decimal Max = 0;
                int Total = 0;
                Student[] studentList = null;

                Console.WriteLine("For how many student's record you want to enter: ");
                string n = Console.ReadLine();

                if (Int32.TryParse(n, out N))
                {

                    studentList = new Student[N];
                    for (int i = 0; i < N; i++)
                    {
                        Console.WriteLine("For student- " + (i + 1) + ", Enter Name: ");
                        string name = Console.ReadLine();

                        Console.WriteLine("For student " + (i + 1) + "Enter Marks: ");
                        string sMrks = Console.ReadLine();
                        int marks = 0;

                        Int32.TryParse(sMrks, out marks);

                        studentList[i] = new Student(name, marks);

                        //Deciding grades (Criteria: 0-49 = F, 50-64 = C, 65-79 = B, 80-100 = A)

                        if (studentList[i].iMarks <= 49)
                        {
                            studentList[i].sGrade = "F";
                        }
                        else if (studentList[i].iMarks <= 64)
                        {
                            studentList[i].sGrade = "C";
                        }
                        else if (studentList[i].iMarks <= 79)
                        {
                            studentList[i].sGrade = "B";
                        }
                        else if (studentList[i].iMarks <= 100)
                        {
                            studentList[i].sGrade = "A";
                        }

                        // Calculating Total
                        Total = Total + studentList[i].iMarks;

                        // Calculating Minimum
                        if (studentList[i].iMarks < Min)
                        {
                            Min = studentList[i].iMarks;
                        }

                        // Calculating Maximum
                        if (studentList[i].iMarks > Max)
                        {
                            Max = studentList[i].iMarks;
                        }

                    }

                }

                if (N > 0)
                {
                    Console.WriteLine("\n\nName\t Marks\t Grade");
                    for (int i = 0; i < N; i++)
                    {
                        Console.WriteLine(studentList[i].sName + "\t" + studentList[i].iMarks + "\t" + studentList[i].sGrade);
                    }

                    Console.WriteLine();

                    Avg = Total / N;
                    Console.WriteLine("Average = " + Avg);
                    Console.WriteLine("Minimum = " + Min);
                    Console.WriteLine("Maximum = " + Max);
                }

                Console.ReadLine();
            }
            catch (Exception exc)
            {
                Console.WriteLine(exc.Message);
                Console.ReadLine();
            }
        }
    }
}