Find the sum of all the multiples of 3 or 5 below 1000

Question

If we list all the natural numbers below 10 that are multiples of 3 or 5, we get 3, 5, 6 and 9. The sum of these multiples is 23. I have the following code but the answer do

Answer 1

Interesting fact of the difference in time between this 2 methods... The second method just calculate only with needed numbers and dont iterate through a loop. Example: 3 * (1+2+3+4...333) (Because 3 * 333 = 999 and 999 < 1000.

   static void Main(string[] args)
    {
        Stopwatch sw = new Stopwatch();

        Calc calculator = new Calc();
        long target = 999999999;
        sw.Start();
        Console.WriteLine("Result: " +  (calculator.calcMultipleSumFast(3,target) + calculator.calcMultipleSumFast(5, target) - calculator.calcMultipleSumFast(15, target)));
        sw.Stop();
        Console.WriteLine("Runtime: " + sw.Elapsed);
        Console.WriteLine();
        sw.Start();
        Console.WriteLine("Result: " + (calculator.calcMultiplesSum(3, 5,1000000000)));
        sw.Stop();
        Console.WriteLine("Runtime: " + sw.Elapsed);

        Console.ReadKey();
    }

    public class Calc
{
    public long calcMultiplesSum(long n1, long n2, long rangeMax)
    {
        long sum = 0;
        for (long i = 0; i < rangeMax; i++)
        {
            if ((i % n1 == 0) || (i % n2 == 0))
            {
                sum = sum + i;
            }
        }
        return sum;
    }

    public long calcMultipleSumFast(long n, long rangeMax)
    {
        long p = rangeMax / n;

        return (n * (p * (p + 1))) / 2;
    }
}