using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;

namespace ConsoleApplication2
{
    internal class Program
    {
        public static void Main(string[] args)
        {
            var dateTime = new DateTime();


            var dateTimes = new List<DateTime>();
            dateTimes.Add(Convert.ToDateTime("2019-05-05"));
            dateTimes.Add(Convert.ToDateTime("2018-01-05"));
            dateTimes.Add(Convert.ToDateTime("2020-02-21"));

            var SredniRok = dateTimes.Select(x => x.Year).Average();
            var IloscPiatkow = dateTimes.Count(x => x.DayOfWeek == DayOfWeek.Friday);

            Console.WriteLine("sredni rok to : " + SredniRok);
            Console.WriteLine("ilosc piatkow to : " + IloscPiatkow);
        }
    }
}