using System;

​

public class Program

{

    static private ulong fact(ulong a) 

    {

        ulong factorial = 1;

        ulong i = 1;

        do

        {

            factorial = factorial * i;

            i++;

        }

        while (i <= a);

        return factorial;

    }   

    static private ulong npr(ulong a, ulong b)

    {

        ulong afact = fact(a);

        ulong abfact = fact(a-b);

        return afact/abfact;

    }

    static private double mean(double a, double b)

    {

        return (a+b)/2;

    }   

    static private double DectoPer(double a) 

    {

        return a * 100;

    }

​

    static private double PertoDec(double a)

    {

        return a / 100;

    }

​

    public static void Main()

    {   string restart= "This variablle will let you quit the program.";

        while(restart != "q")

        {

            start:

            Console.Clear();

        Console.WriteLine("Please choose which option you would like to calculate");

        Console.WriteLine("Press 1 for factorial, 2 to convert from decimal to percent, 3 to convert from percent to decimal, 4 for arrange and 5 for find mean.");

        string choice = Console.ReadLine();

        if (choice == "1")

        { Console.WriteLine("Please enter the factorial");

            ulong n = ulong.Parse(Console.ReadLine());

         Console.WriteLine( "The factorial is: " + fact(n));

        }

        else if (choice == "2")

        {Console.WriteLine("Please enter the decimal that you would like to convert.");

            double n = double.Parse(Console.ReadLine());

         Console.WriteLine( "The decimal converted to percent is: " + DectoPer(n));

        }

        else if (choice == "3")

        {Console.WriteLine("Please enter the percent that you would like to convert.");

            double n = double.Parse(Console.ReadLine());

         Console.WriteLine( "The percent converted to decimal is: " + PertoDec(n));

         }

        else if ( choice == "4")

        { Console.WriteLine("Please enter your total # of items.");

            ulong n = ulong.Parse(Console.ReadLine());

         Console.WriteLine("Please enter the number you would like to arrange.");

         ulong r = ulong.Parse(Console.ReadLine());

         Console.WriteLine( "The numbers of your arrange is: " + npr(n,r));

        }

        else if ( choice == "5")

        { Console.WriteLine("Please enter your one number");

            double n = double.Parse(Console.ReadLine());

         Console.WriteLine("Please enter your other the number.");

         double r = double.Parse(Console.ReadLine());

         Console.WriteLine( "The mean of your numbers is: " + mean(n,r));

    }

                Console.WriteLine("Press q to quit the progress or any other key to continue.");

            restart = Console.ReadLine();

        }

}

}