Official explanation:
When the GMAT asks abstract calculation problems, your job is to make that abstraction more concrete using two methods:
1) Limit the number of options by adhering to and proving rules about the situation provided.
2) Test numbers via trial-and-error to eliminate options and to learn more about the situation.
For example here, you can start by limiting the options for A: since A must be even and it cannot be 0 (if it were, then the number ADD would just be DD), you only have four options for A: 2, 4, 6, and 8.
But if you quickly try 2, you'll see that even with the greatest possible D, you won't have large enough numbers to produce a thousands digit S in the sum. 299 + 299 + 299 is 897, and you need a number that's 1000 or greater. So you can limit your options for A to 4, 6, or 8.
Next, consider the sum SUMS. Since you're adding three three-digit numbers to produce SUMS, the S has a limit to it also. Even if you added the three greatest three-digit numbers possible, 999 + 999 + 999, you'd end up with a number less than 3000. So S can only be 1 or 2.
Also consider that in SUMS M and S must be different digits, meaning that adding three Ds must sum to something greater than 10 so that the operation forces you to carry a tens digit and make M different from S. (For example, 411 + 411 + 411 would give you 1233 with the same M and S. You need D to be large enough that you don't have repeat digits in the tens and units places in the sum).
So from quick trial and error and some application of the rules provided, you know three things:
-A can only be 4, 6, or 8 -S can only be 1 or 2 -D must be greater than 3
From here you can use some units digit rules along with trial-and-error to arrive at SUMS. In the units place, 3D (the sum of D + D + D) can only be 1 or 2. In order for it to be 1, D would have to be 7. So you might try:
477 + 477 + 477 = 1431
But note that in this situation U and D are each 4, which violates the situation that they must be different values. So this cannot work. The next possible value ending in 77 would be 677, but at that point the sum would begin with a 2 (677 + 677 + 677 = 2031, or you could just know that 667 is 1/3 of 2000 and so three 677s would be greater than that). And that doesn't work because the S values in SUMS would be different.
So S cannot be 1, meaning that it must be 2. In order for that to be the case, you'd need D to be 4 (since 4x3 = 12). Here you can try again: if A cannot be 4 (that would be a repeat value), then you could try 6, but recognize again that you'd need something greater than 666 to reach a thousands digit of 2, since 2000 divided by 3 is 666.67. So your only choice is 844 + 844 + 844 = 2532. This then means that the correct answer is 5.