List<int> notPrime = new List<int>();

        // sieve of Eratosthenes - excludes gradually all numbers which can be divided by 2, 3, 5, 7
        // you can confirm this with the y locals values in debug mode
        // I have chosen this loop solution, because the loop length can be easily ammended
        // to include any bigger range of numbers

        for (int x = 2; x < 100; x++)
        {
            for (int y = x * 2; y < 100; y = y + x)
            {

                if (!notPrime.Contains(y))
                {
                    notPrime.Add(y);
                }

            }

        }