using System;

​

public class Program

{

    static private ulong fact(ulong a) //factorial

    {

        ulong factorial = 1;

        ulong i = 1;

        do

        {

            factorial = factorial * i;

            i++;

        }

        while (i <= a);

        return factorial;

    }

​

    static private double DectoPer(double a) //decimal to percent

    {

        return a * 100;

    }

​

    static private double PertoDec(double a) //percent to decimal

    {

        return a / 100;

    }

    static private double nPr(ulong n,ulong r)// nPr

    {

        ulong nfact = fact(n);

        ulong nrfact = fact(n-r);

        return nfact/nrfact;

    }

    static private double nCr(ulong n,ulong r)// nCr

    {

        ulong nfact = fact(n);

        ulong nrfact = fact(n-r);

        ulong nrrfact = nfact/nrfact;

        ulong rfact = fact(r);

        return nrrfact * rfact;

    }

    public static void Main()

    {

        start:

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

        Console.WriteLine("Press 1 for factroial, 2 to convert from decimal to percent, 3 to convert from percent to decimal, 4 to calcualte premutation, 5 to calculate combination");

        string choice = Console.ReadLine();

        if (choice == "1")

        {

            Console.WriteLine("What would you like to calculate the factorial of?");

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

            Console.WriteLine("The factorial of  " + ans + " = " + fact(ans));

        }

        else if (choice == "2")

        { Console.WriteLine("What decimal do you need to be a percent?");

         ulong ans2 = ulong.Parse(Console.ReadLine());

        Console.WriteLine(ans2+" in percent form is- " + DectoPer(ans2));

        }

        else if (choice == "3")

        { Console.WriteLine("What percent do you want to convert to decimal?");

         ulong ans3 = ulong.Parse(Console.ReadLine());

        Console.WriteLine(ans3+" in decimal form is- " + PertoDec(ans3));

        }

        else if (choice == "4")

        {

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

        }

        else

        { Console.WriteLine("Please enter a valid number thank you!");

         goto start;

        }

    }

}