If n is a positive integer, is n³ - n divisible by 4 ?
(1) n = 2k + 1, where k is an integer.
(2) n² + n is divisible by 6.
Target question:
Is n³ - n divisible by 4?
This is a great candidate for rephrasing the target question.
Aside: Rephrasing the target question can often make data sufficiency questions easier (and faster) to solve. We have a video on this strategy:
https://www.gmatprepnow.com/module/gmat- ... cy?id=1100
Notice that we can take n³ - n and factor it to get n(n² - 1), which equals n(n-1)(n+1) or (n-1)(n)(n+1)
Now recognize that n-1, n, and n+1 are three consecutive integers.
The GMAT often hides this kind of information within given algebraic expressions.
So, at this point, we can rephrase the target question as:
Is the product of 3 consecutive integers divisible by 4?
If we dig a little deeper, we can further rephrase the target question to make the question even easier to solve.
To do this, we'll ask, "Under what circumstances is the product of 3 consecutive integers divisible by 4? Well, there are two such circumstances.
Circumstance 1: The first and last integers are even. For example, the product of 2, 3, and 4 will be divisible by 4. In this circumstance, the middle number (n) is odd.
Circumstance 2: The middle integer is divisible by 4. For example, the product of 7, 8, and 9 must be divisible by 4 since the number 8 is already divisible by 4. In this circumstance, the middle number (n) is divisible by 4.
Given these two circumstances, we can rephrase the target question as:
Is n either odd or divisible by 4?
At this point, we can check the statements.
Statement 1: n = 2k + 1, where k is an integer
This is a very clever way of telling us that n is odd. In fact, this is the formal definition of an odd number.
Since n is odd, we can now answer the
rephrased target question with certainty.
So, statement 1 is SUFFICIENT
Statement 2: n² + n is divisible by 6
Notice that we can take n² + n and rewrite it as (n)(n+1), and we know that n and n+1 are two consecutive integers.
This information yields different possible cases, here are two.
case a: n=2, n+1=3, in which case
n is neither odd nor divisible by 4
case b: n=3, n+1=4, in which case
n is odd
Since statement 2 yields conflicting answers to our
rephrased target question, it is NOT SUFFICIENT.
Answer =
A
Cheers,
Brent
