If you don't want to add them one by one, you can use the formula for the sum of an arithmetic series.
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The prime factors of 200 are 2 x 5 x 2 x 5 x 2. All possible multiples of those are all of 200's divisors. If you calculate all the divisors and add them up you will get the sum of divisors for 200. If that sum is greater than 400, then 200 is abundant. Here are a few examples of 200's divisors: 200, 100, 2, 50, 4, 8, 25. Already those add up to 391... if there are any more then 200 is certainly abundant. Does 200 have any other divisors?
Look at the multiples of 6. The smallest and greatest multiples of 6 that can be divided evenly by 4 are 12 and 48. So the common multiples of 4 and 6 up to 50 are 12, 24, 36, and 48.
There are 200 of them. Starting with 45*23 = 1035 and all the multiples of 45 up to 45*222 = 9990.
4, 8, 12, 16, 20, 24, 28, 32, 36, 40 and just keep adding 4 until you get to 200.
The numbers up to 12 are 4,8,12,16,20,24,28,32,36,40,44,48.