Last stretch

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApp1
    {
        class Program
        {

            static void CalcMin(Decimal[] grades)
            {
                grades.Min().ToString();
                Console.WriteLine("the minimum grade is: " + grades.Min());
                return;
            }

            static void CalcMax(Decimal[] grades)
            {
                grades.Max().ToString();
                Console.WriteLine("the Maximum is: " + grades.Max());
                return;
            }

            static decimal SumGrades(Decimal[] grades)
            {

                decimal Sum = 0;
                foreach (Decimal singlegrade in grades)
                {
                    Sum = Sum + singlegrade;
                }
                Console.WriteLine("The sum of all the grades is: " + Sum);
                return Sum;
            }

            static Decimal Calcaverage(Decimal Sum, int num)
            {
                Decimal average = (Sum / num);
                Console.WriteLine("the average of the grades is: " + average);
                return average;
            }

            static decimal CalcFailed(Decimal[] grades, int num)
            {

                decimal fail = 0;
                for (int i = 0; i < num; i++)
                {
                    if (grades[i] < 50)
                    {
                        Console.WriteLine();
                        fail = fail + 1;
                    }
                }
                Console.WriteLine("the number of students who failed is: " + fail);
                return fail;
            }

            static decimal CalcPassed(Decimal[] grades, int num)
            {
                decimal pass = 0;

                for (int i = 0; i < num; i++)
                {
                    if (grades[i] >= 50)
                    {
                        Console.WriteLine();
                        pass = pass + 1;
                    }
                }

                Console.WriteLine("the number of students who passed is: " + pass);
                return pass;
            }

            static decimal Failedpercent(decimal fail, int num)
            {
            decimal percentfail = fail / num;
                Console.WriteLine("the percentage of students who failed is: " + percentfail);
                return percentfail;
            }

            static decimal Passpercent(decimal pass, int num)
            {
                Decimal percentpass = pass / num;
                Console.WriteLine("the percentage of the perople who passed is: " + percentpass);
                return percentpass;
            }

            static Tuple<string[], string[], string[], string[], int> Enterinfo()

            {
                Console.WriteLine("How many students do you have? ");

                int num = int.Parse(Console.ReadLine());
                string[] snum = new string[num];

                string[] lastnames = new string[num];

                string[] firstnames = new string[num];

                string[] grades = new string[num];


                for (int i = 0; i < num; i++)

                {

                    Console.WriteLine("What is student #" + (i + 1) + "'s Student ID number? ");

                    snum[i] = Console.ReadLine();

                    Console.WriteLine("What is student #" + (i + 1) + "'s first name? ");

                    firstnames[i] = Console.ReadLine();

                    Console.WriteLine("What is student #" + (i + 1) + "'s last name? ");

                    lastnames[i] = Console.ReadLine();

                    Console.WriteLine("What is student #" + (i + 1) + "'s grade? ");

                    grades[i] = Console.ReadLine();


                }
                Console.WriteLine(".............");
                for (int i = 0; i < num; i++)
                {
                    Console.WriteLine("student #" + (i + 1));
                    Console.WriteLine(firstnames[i] + " ");
                    Console.WriteLine(lastnames[i] + " ");
                    Console.WriteLine(snum[i] + " ");
                    Console.WriteLine(grades[i] + " ");
                }
                Console.WriteLine();
                var Input = Tuple.Create<string[], string[], string[], string[], int>(snum, lastnames, firstnames, grades, num);
                return Input;

            }




            static string[] Statistics(string[] snum, Decimal[] grades, int num)
            {

                CalcMin(grades);
                CalcMax(grades);
                Calcaverage(SumGrades(grades), num);
                Failedpercent(CalcFailed(grades, num), num);
                Passpercent(CalcPassed(grades, num), num);
                return null;
            }
            static Tuple<string[], string[], string[], Decimal[], int> Changeinfo(string[] snum, string[] firstnames, string[] lastnames, Decimal[] grades, int num)
            {

            Console.WriteLine("Please enter which Student number you want to find?");
            string sd = Console.ReadLine();
            for (int i = 0; i < num; i++)

            {
                if (sd == snum[i])

                {

                    Console.WriteLine("Student ID: " + snum[i] + "Last Name: " + firstnames[i] + "First name: " + lastnames[i] + "Current Grade: " + grades[i].ToString() + " ");

                    Console.WriteLine("What is student #" + snum[i] + "'s first name? ");

                    firstnames[i] = Console.ReadLine();

                    Console.WriteLine("What is student #" + snum[i] + "'s last name? ");

                    lastnames[i] = Console.ReadLine();

                    Console.WriteLine("What is student #" + snum[i] + "'s grade? ");

                    grades[i] = Decimal.Parse(Console.ReadLine());

                    Console.Clear();

                    break;

                }

            }
            Console.ReadLine();
            var Change = Tuple.Create<string[], string[], string[], Decimal[], int>(snum, lastnames, firstnames, grades, num);
            return Change;
            }

            private static void Main(string[] args)
            {
                char answer;

                Console.WriteLine("List of possible tasks");

                Console.WriteLine("Enter new information. Press 1");

                Console.WriteLine("Print statistics. Press 2");

                Console.WriteLine("Change entered information. Press 3");
                string[] snum = new string[1];

                string[] lastnames = new string[1];

                string[] firstnames = new string[1];

                Decimal[] grades = new Decimal[1];

                int num = 0;

                do
                {
                    Console.Write("Please select a task");
                    string x = Console.ReadLine();

                    switch (x)

                    {

                        case "1":
                            Tuple<string[], string[], string[], string[], int> returned = Enterinfo();
                        snum = returned.Item1;
                        firstnames = returned.Item2;
                        lastnames = returned.Item3;
                        grades = Array.ConvertAll(returned.Item4, Decimal.Parse);
                        num = returned.Item5;

                        break;

                        case "2":

                            Statistics(snum, grades, num);

                            break;

                        case "3":
                        Tuple<string[], string[], string[], Decimal[], int> changed = Changeinfo(snum, firstnames, lastnames, grades, num);
                        snum = changed.Item1;
                        firstnames = changed.Item2;
                        lastnames = changed.Item3;
                        grades = changed.Item4;
                        num = changed.Item5;

                            break;

                        default:

                            Console.WriteLine("Invalid response.");

                            break;

                    }

                    Console.Write("Do you want to issue an addistion action");

                    answer = Char.Parse(Console.ReadLine());

                } while (answer == ('y' | 'Y'));

                Console.WriteLine("Thank you have a good day!");

                Console.ReadLine();
            }
        }
    }