thing to do:
- find all the mutiples of 3 that are smaller then 1000
- find all the multiples of 5 that are smaller then 1000
- add all the number you've found
- done

You might want to look at the modules (%) operator for this assignment

Start by figuring out how to get the 3, 5, 6 and 9. If you know how to do that them its easy to find all those numbers below 1000. If you can't do it in your head then write it out with pencil & paper. Think about the process that is needed to get those numbers. Hint: use the mod operator % to determine if a number is evenly divisible by either 3 or 5 and a loop to test each number between 1 and 1000.