Project Euler #1: Multiples of 3 and 5

Hi guys, today I will show you how to solve Project Euler Problem 1, the question is

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.

Find the sum of all the multiples of 3 or 5 below 1000000000.

I tried to solve this question using for loop. It was working fine but it was taking so many times to get the result. Then I implemented some mathematical logic to solve this issue.

static void Main(String[] args) {             int i = Convert.ToInt32(Console.ReadLine());             List<long> list = new List<long>();             for (int j = 0; j < i; j++)             {                 list.Add(Convert.ToInt64(Console.ReadLine()));             }             foreach(long k in list)             {                 long sum=0;                 long i3 = (((k - 1) / 3) * (((k - 1) / 3) + 1)) / 2;                 sum += i3 * 3;                 long i5 = (((k - 1) / 5) * (((k - 1) / 5) + 1)) / 2;                 sum += i5 * 5;                 long i15 = (((k - 1) / 15) * (((k - 1) / 15) + 1)) / 2;                 sum -= i15 * 15;                 Console.WriteLine(sum);             }     }