From the question stem, we see our set P looks something like this: {...a, b, c, d, 3, e, f...}. That's my way of denoting that there is the number 3, and then some other numbers. Some may be bigger, some may be smaller... but right now we don't know. We also don't know if the set is finite or infinite.
Coming to statement (1), we see that for any number in the set P, the number + 3 is also in the set P. Right now, the only number that we know is in the set is 3. So, 3 + 3 = 6 must also be in the set. Since 6 is in the set, then 6 + 3 = 9 must also be in the set. You can see where this is going. The set {3, 6, 9, 12, ....} must be a subset of P. I'm careful by saying subset, because there is nothing that would prevent, say, 7 from being in P (and thus 10, 13, 16, etc.)
Ok, so from statement (1) we see that P has to contain every positive multiple of 3. Thus, statement (1) is sufficient.
Coming to statement (2), we see that for any number in the set P, the number - 3 is also in the set P. Right now, the only number that we know is in the set is 3. So, 3 - 3 = 0 must also be in the set. Since 0 is in the set, then 0 - 3 = - 3 must also be in the set. You can see where this is going. The set {3, 0, -3, -6, -9, ....} must be a subset of P. This does NOT guarantee that all positive multiples of 3 are in the set. It only makes a guarantee about the NEGATIVE multiples and 3 itself. So, statement (2) is not sufficient.
Since statement (1) is sufficient and statement (2) is not, the answer is A.
Hope that helped,
Tatiana
Tatiana Becker | GMAT Instructor | Veritas Prep
