If \(n\) is a positive integer and \(r\) is the remainder

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If \(n\) is a positive integer and \(r\) is the remainder when \(n^2-1\) is divided by \(8\), what is the value of \(r\)?

1) \(n\) is odd.
2) \(n\) is not divisible by \(8\).

OA A

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by Brent@GMATPrepNow » Fri Mar 29, 2019 5:54 am

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AAPL wrote:Manhattan Prep

If \(n\) is a positive integer and \(r\) is the remainder when \(n^2-1\) is divided by \(8\), what is the value of \(r\)?

1) \(n\) is odd.
2) \(n\) is not divisible by \(8\).

OA A
Given: r is the remainder when (n² - 1) is divided by 8

Target question: What is the value of r?

Statement 1: n is odd
Let's test some ODD values of n
If n = 1, then n² - 1 = 1² - 1 = 0, and 0 divided by 8 leaves remainder 0. So, the answer to the target question is r = 0
If n = 3, then n² - 1 = 3² - 1 = 8, and 8 divided by 8 leaves remainder 0. So, the answer to the target question is r = 0
If n = 5, then n² - 1 = 5² - 1 = 24, and 24 divided by 8 leaves remainder 0. So, the answer to the target question is r = 0
If n = 7, then n² - 1 = 7² - 1 = 48, and 0 divided by 8 leaves remainder 0. So, the answer to the target question is r = 0
At this point, we might conclude that r will ALWAYS be 0
So, statement 1 is SUFFICIENT

----ASIDE--------------------------------
If you're not convinced, here's an algebraic solution as well:
If n is ODD, then n = 2k + 1 (for some integer value of k)
So, n² - 1 = (2k + 1)² - 1 = 4k² + 4k + 1 - 1 = 4k² + 4k = 4(k² + k)

Notice that, if k is odd, then k² + k is EVEN, which means k² + k = 2 times some integer
So, n² - 1 = 4(k² + k) = 4(2 times some integer) = 8 times some integer
In other words, n² - 1 is a multiple of 8, which means the answer to the target question is r = 0

Similarly, if k is even, then k² + k is EVEN, which means k² + k = 2 times some integer
So, n² - 1 = 4(k² + k) = 4(2 times some integer) = 8 times some integer
In other words, n² - 1 is a multiple of 8, which means the answer to the target question is r = 0

In both cases, the answer to the target question is r = 0
Since we can answer the target question with certainty, statement 1 is SUFFICIENT
------------------------------------------

Statement 2: n is not divisible by 8
There are several values of n that satisfy statement 2. Here are two:
Case a: n = 3. In this case, n² - 1 = 3² - 1 = 8, and 8 divided by 8 leaves remainder 0. So, the answer to the target question is r = 0
Case b: n = 4. In this case, n² - 1 = 4² - 1 = 15, and 15 divided by 8 leaves remainder 7. So, the answer to the target question is r = 7
Since we cannot answer the target question with certainty, statement 2 is NOT SUFFICIENT

Answer: a

Cheers,
Brent
Brent Hanneson - Creator of GMATPrepNow.com
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