在一堆数中查询相加得某个数的组合

如题，一个网友问的，我又去问其他的网友，再网上搜索，发贴问，最后终于解决，不过目前还没有明白为什么要这样写，以下是代码：

  class Program
    {
        static void Main(string[] args)
        {
            double[] numbers = new double[] { 258.5, 1229.1, 39.6, 660.3, 660, 660, 165, 165, 3060, 270, 4.8, 440, 279.06, 47.89, 38.02, 600, 286, 63.36, 1530, 1320, 1344, 3954 };




            Console.WriteLine("原数列：258.5,1229.1,39.6,660.3,660,660,165,165,3060,270,4.8,440,279.06,47.89,38.02,600,286,63.36,1530,1320,1344,3954\n\n");


             SumFinder.FindSum(numbers, 7785.52);
            // SumFinder.FindSum(new double[]{1,2,2,3,5}, 10);


        }
    }




    static class SumFinder
    {
        public static void FindSum(double[] numbers, double targetSum)
        {
            int elementCount = numbers.Length;
            int lastCombination = 2 << elementCount - 1;
            int firstCombination = 1;
            double sum;
            for (int i = firstCombination; i <= lastCombination; i++)
            {
                sum = 0;
                for (int pos = 0; pos < elementCount; pos++)
                {
                    if ((i & (1 << pos)) != 0) //当前组合第pos个数是否取中
                        sum += numbers[pos];
                }
                if (sum == targetSum)
                {
                    string str = "";
                    for (int pos = 0; pos < elementCount; pos++)
                    {
                        if ((i & (1 << pos)) != 0)
                        {
                           str += numbers[pos]+" + ";
                        }
                    }
                    Console.Write(str.Substring(0,str.Length-3)+" = " + targetSum);
                    Console.WriteLine();
                }
            }
            Console.WriteLine("结束");
            Console.ReadLine();
        }

    }