Alex

using System;
using System.Threading;

namespace ConsoleApp19
{
    class Program
    {
        public static int[] A;
        public static int countNumbers;
        public static int countThread;
        public static object Locker = new object();
        public static int subArrayLen;
        public static int result = 0;

        static void Main(string[] args)
        {
            Console.Write("Количество цифр: ");

            if (int.TryParse(Console.ReadLine(), out countNumbers) == false || countNumbers < 1)
            {
                countNumbers = 10;
                Console.WriteLine($"Нельзя такое писать,количество чисел - {countNumbers}");
            }
            A = new int[countNumbers];
            Console.WriteLine("Задать числам значение:");
            UserFillA(ref A);
            Console.WriteLine("A: ");

            for (int i = 0; i < countNumbers; i++)
                Console.Write($"{A[i]} ");
            Console.Write("\nКоличество потоков: ");

            if (int.TryParse(Console.ReadLine(), out countThread) == false || countThread < 1)
            {
                countThread = 1;
                Console.WriteLine($"Нельзя такое писать, количество потоков - {countThread}");
            }

            if (countThread > countNumbers)
            {
                countThread = countNumbers;
                Console.WriteLine("Слишком много потоков, количество потоков - {count}");
            }
            subArrayLen = countNumbers / countThread + 1;
            Thread[] threads = new Thread[countThread];
            StartAllThreads(ref threads);
            WaitAllThreads(ref threads);
            Console.WriteLine($"Количество элементов кратных пяти: {result}");
            Console.ReadLine();
        }

        private static void WaitAllThreads(ref Thread[] threads)
        {
            foreach (Thread item in threads)
            {
                Console.WriteLine($"{item.Name} - enabled");
                item.Join();
                Console.WriteLine($"{item.Name} - switched off");
            }
        }

        private static void StartAllThreads(ref Thread[] threads)
        {
            for (int i = 0; i < countThread; i++)
            {
                PartArray partArray = new PartArray
                {
                    StartIndex = i * subArrayLen,
                    EndIndex = (i + 1) * subArrayLen
                };

                if (partArray.EndIndex > countNumbers)
                    partArray.EndIndex = countNumbers;
                threads[i] = new Thread(() => SumWith3(partArray));
                threads[i].Name = $"Поток№{i + 1}";
                threads[i].Start();
            }
        }

        private static void UserFillA(ref int[] array)
        {
            for (int i = 0; i < array.Length; i++)
            {
                Console.Write($"{i + 1}:");

                if (int.TryParse(Console.ReadLine(), out array[i]) == false || array[i] < 1)
                {
                    Console.WriteLine("Неправильный ввод");
                    i--;
                }
            }
        }

        private static void SumWith3(PartArray partArray)
        {
            int start = partArray.StartIndex;
            int end = partArray.EndIndex;

            for (int i = start; i < end; i++)
            {
                if (A[i] % 5 == 0)
                    lock (Locker)
                        result++;
            }
        }
    }

    class PartArray
    {
        public int StartIndex;
        public int EndIndex;
    }
}