Problem 1: Add all the natural numbers below one thousand that are multiples of 3 or 5.

Problem: 

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 1000.

First, I use the C# to solve this one:

using System;

 class ProblemOne
{
    static void Main()
    {
        int numThousand = 1000;
        int sum = 0;

        for(int i = 0; i < numThousand; ++i)
            if(i % 3 == 0 || i % 5 == 0)
                sum += i;
        Console.WriteLine("Sum of these multiples is {0}", sum);

    }
}
 

secondly, consider the C++ code:

#include<iostream>
using namespace std;

int main()
{
	int numOneThousand = 1000;
	int sum = 0;

	for(int i = 0; i < numOneThousand; ++i)
		if(i % 3 == 0 || i % 5 == 0)
			sum += i;
	cout<< "Sum below 1000 is:" << sum << endl;
}

Last but not the least, Python:

sum = 0
for i in range(1000):
    if i % 3 == 0 or i % 5 == 0:
        sum += i
print sum

answer:233168