Methods Menu Exercise (temp)

using System;
using System.Collections.ObjectModel;
using System.Linq;
using System.Linq.Expressions;
using System.Threading;

namespace ConsoleTests
{
    class ConsoleTests
    {
        static void Main()
        {
            Console.WriteLine("Available choices: ");
            Console.WriteLine();
            Console.WriteLine("1. Put the digits from an integer number into a reversed order");
            Console.WriteLine("2. Calculate the average of given sequence of numbers");
            Console.WriteLine("3. Solve the linear equation a * x + b = 0");
            Console.Write("\nEnter your choice: ");
            byte answer = byte.Parse(Console.ReadLine());
            if (answer == 1)
            {
                int integer = 0;
                bool isValid;
                bool isValidInt32;
                do
                {
                    Console.Write("\nEnter valid integer (1 to 50 000 000): ");
                    //int test;
                    isValidInt32 = int.TryParse(Console.ReadLine(), out integer);
                    isValid = integer >= 1 && integer <= 50000000;
                } while (isValid == false);
                Console.WriteLine("\n-> Reversed: {0}", ReverseDigits(integer));
            }
            else if (answer == 2)
            {
                string collection;
                bool isNotEmpty;
                do
                {
                    Console.Write("\nEnter numbers (separated by space): ");
                    collection = Console.ReadLine();
                    isNotEmpty = collection.Length >= 1;
                } while (isNotEmpty == false);
                string[] numsArray = collection.Split(' ').ToArray();
                Console.WriteLine("-> Average: {0}", CalcAverage(numsArray));
            }
            else if (answer == 3)
            {
                decimal a;
                bool isNotZero;
                do
                {
                    Console.Write("Enter a (a != 0): ");
                     a = decimal.Parse(Console.ReadLine());
                    isNotZero = a != 0;
                } while (isNotZero == false);

                Console.Write("Enter b: ");
                decimal b = decimal.Parse(Console.ReadLine());
                CalcLinearEquation(a,b);
            }

        }

        static string ReverseDigits(int a)
        {
            int[] digits = a.ToString().ToCharArray().Select(x => (int)Char.GetNumericValue(x)).ToArray();
            Array.Reverse(digits);
            string output = string.Join("", digits);
            return output;
        }

        static decimal CalcAverage(params string[] array)
        {
            decimal sum = 0;
            foreach (var num in array)
            {
                sum += decimal.Parse(Convert.ToString(num));
            }
            decimal average = sum/array.Length;
            return average;
        }

        static void CalcLinearEquation(decimal a, decimal b)
        {
            decimal x = -b/a;
            Console.WriteLine("x = {0}", x);
            Console.WriteLine("{0}*({1})+{2}=0", a, x, b);
        }
    }
}