Anyone want to Address this problem?

Hi bpolley00,

For these types of questions, it's important to pay attention to the language. The prompt asks which of the following MUST also be in the set? Not what "could" be in the set, what "must" be in the set (according to whatever rules we've been given to work with).

We're told that if X is in the set, then (X-3) is also in the set. This tells us that the set is infinite (once we have a number, we would subtract 3 from that number, then from the next number and so on). We also know that our math moves "forwards", so we can't work backwards from a value, we can only move forwards (by subtracting 3, because that's the only rule we have to work with).

Since we're told that 1 is in the set, we can move forwards from there (by subtracting 3, as we're told to do).

Thus, we know that these numbers MUST be in the set:

1, -2, -5, -8, -11, -14, etc.

Other numbers could be in the set, but we don't know if they must be in the set.

Thus, Roman Numeral III is the only value that we know for sure MUST be in the set.

Final Answer: C

GMAT assassins aren't born, they're made,
Rich

Hi BP,

I understand your frustration, so maybe I can rephrase the idea.

I think that you can agree that since the number 1 is in the set, then the number -2 is also in the set.

Since the number -2 is in the set, what MUST also be in the set? Think about what the prompt tells you (If X is in the set, then X-3 is also in the set). So, we could NOW say that if X = -2, then -5 MUST also be in the set.

Going on from there, if X = -5, then -8 MUST also be in the set, and so on.

So even though the prompt doesn't state that the set is infinite, the one rule that we have to work with means that the set will go on forever (with increasingly negative values).

GMAT assassins aren't born, they're made,
Rich