On the GMAT, problems about multiples are generally constrained to POSITIVE multiples.
I've amended the problem to reflect how it would appear on the GMAT.
kris610 wrote:
If S is a sequence of consecutive positive multiples of 3, how many multiples of 9 are there in S?
(1) There are 15 terms in S.
(2) The greatest term of S is 126.
A
Statement 1: There are 15 terms in S.
Among every 3 consecutive multiples of 3, exactly ONE will be a multiple of 9:
3,6,
9
6,
9,12
9,12,15
12,15,1
8
15,
18,21
18,21,24
The 15 terms in S = 5 cycles of 3 consecutive multiples of 3.
Within each cycle, there will be exactly ONE multiple of 9.
Thus, there will be 5 multiples of 9 in set S.
SUFFICIENT.
Statement 2: The greatest term of S is 126.
Without knowing the total number of terms, we can't determine how many multiples of 9 are in set S.
INSUFFICIENT.
The correct answer is
A.
Learn to recognize how the GMAT lays TRAPS.
Virtually every test-taker would recognize that given the number of terms (statement 1) and the greatest term (statement 2), we could simply count how many multiples of 9 are in set S.
If the OA is C, the question becomes TOO EASY.
Thus, the OA is probably
NOT C.
Whenever C seems too obvious an answer choice, be suspicious: there is a good chance that ONE of the two statements will be SUFFICIENT on its own.
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