If n is an integer, is n - 1 even?
(1) n - 2 is odd
(2) n + 1 is even
[spoiler]OA=D[/spoiler]
Why is sufficient the first statement? May anyone help me? Thanks in advanced.
If n is an integer, is n - 1 even?
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If n-1 is even, we get the following options for n-1:Gmat_mission wrote:If n is an integer, is n - 1 even?
(1) n - 2 is odd
(2) n + 1 is even
n-1 = 0, 2, 4, 6, 8, 10, 12...
Adding 1 to every value in the list above, we get the following options for n:
n = 1, 3, 5, 7, 9, 11, 13....
The resulting list indicates that n-1 will be even if N IS ODD.
Question stem, rephrased:
Is n odd?
Statement 1:
Since n-2 is odd, we get the following options for n-2:
n-2 = 1, 3, 5, 7, 9, 11, 13....
Adding 2 to every value in the list above, we get the following options for n:
n = 3, 5, 7, 9, 11, 13, 15....
Every value in the resulting list in blue is ODD.
Thus, the answer to the rephrased question stem is YES.
SUFFICIENT.
Statement 2:
Since n+1 is even, we get the following options for n+1:
n+1 = 0, 2, 4, 6, 8, 10, 12...
Subtracting 1 from every value in the list above, we get the following options for n:
n = -1, 1, 3, 5, 7, 9, 11....
Every value in the resulting list in blue is ODD.
Thus, the answer to the rephrased question stem is YES.
SUFFICIENT.
The correct answer is D.
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Some important rules:Gmat_mission wrote:If n is an integer, is n - 1 even?
(1) n - 2 is odd
(2) n + 1 is even
1. ODD +/- ODD = EVEN
2. ODD +/- EVEN = ODD
3. EVEN +/- ODD = ODD
4. EVEN +/- EVEN = EVEN
Target question: Is n - 1 even?
Statement 1: n - 2 is odd
In other words, we can write n - 2 = ODD
Add 1 to both sides to get: n - 2 + 1 = ODD + 1
Simplify: n - 1 = ODD + 1
1 is ODD, so we can write: n - 1 = ODD + ODD
From rule #1, we can write: n - 1 = EVEN
So, the answer to the target question is YES, n-1 IS even
Since we can answer the target question with certainty, statement 1 is SUFFICIENT
Statement 2: n + 1 is even
In other words, we can write n + 1 = EVEN
Subtract 2 from to both sides to get: n + 1 - 2 = EVEN - 2
Simplify: n - 1 = EVEN - 2
2 is EVEN, so we can write: n - 1 = EVEN - EVEN
From rule #4, we can write: n - 1 = EVEN
So, the answer to the target question is YES, n-1 IS even
Since we can answer the target question with certainty, statement 2 is SUFFICIENT
Answer: D
Cheers,
Brent