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A Number-Theory Question

Expert replies
by npalani07 » Sat Jan 25, 2014 12:13 pm
If x is an integer, is x³- 3x² + 2x divisible by 4?
(1) x = 3y, where y is an integer
(2) x = 7z, where z is an integer

Ans: A
Source: Derived from a DS problem in MGMAT
If possible, please tell me: What is the likely GMAT-difficulty-level of this Question?
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Source: — Data Sufficiency |

by [email protected] » Sat Jan 25, 2014 6:50 pm
Hi npalani07,

This DS question has a quirky element to it which you really have to deal with first before you deal with the two Facts.

X^3 - 3X^2 + 2X can be factored into....
X(X^2 - 3X + 2) which can be further factored into....
X(X-1)(X-2)

So, the question ultimately asks: Is X(X-1)(X-2) evenly divisible by 4? This is a YES/NO question. We're told that X is an integer.

The "secret" to this question is that if X OR (X-1) OR (X-2) is divisible by 4, then the answer is YES. If NONE of them are divisible by 4, then the answer is NO.

Fact 1: X = 3Y; Y is an integer

This tells us that X is a multiple of 3 (0, 3, 6, 9, etc.)

If X = 0, then (0)(-1)(-2) = 0 and the answer is YES
If X = 3, then (3)(2)(1) = 6 and the answer is NO
Fact 1 is INSUFFICIENT

Fact 2: X = 7Z; Z is an integer

This tells us that X is a multiple of 7 (0, 7, 14, 21, etc.)
If X = 0, then (0)(-1)(-2) = 0 and the answer is YES
If X = 7, then (7)(6)(5) = 210 and the answer is NO
Fact 2 is INSUFFICIENT

Together, we know that X has to be a multiple of 3 AND 7 (0, 21, 42, etc.)
If X = 0, the the answer is YES
If X = 21, then (21)(20)(19) has 20 as a factor, so it IS divisible by 4 (YES answer)
If X = 63, then (63)(62)(61) has NO factor divisible by 4 (NO answer)
Together, INSUFFICIENT

Final Answer: E

These individual concepts can (and likely will) show up on the GMAT, but they're not necessarily going to show up "packaged" this way.

GMAT assassins aren't born, they're made,
Rich
Contact Rich at [email protected]
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by npalani07 » Sun Jan 26, 2014 7:03 am
Hi Rich,
Thanks for your solution.
Question just popped up, and I used 2 numbers (not good samples) for 'y' to prove Stmt(1). Eventually, I see that other numbers for y do not work out.
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