Riddle 1: Multiples of 3 or 5

The Task: 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. As often with these kind of riddles, there is a stra...