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700 Algrbra! Need help again. Thanks so much! [#permalink]
11 Feb 2010, 13:52

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E

Difficulty:

(N/A)

Question Stats:

63% (01:13) correct
38% (01:17) wrong based on 7 sessions

Could anyone explain to me why the number 1 would work in the first situation? I understand why 3, 5, 7 or others work. But why 1 works, too? Thank you so much for this great help!!

Re: 700 Algrbra! Need help again. Thanks so much! [#permalink]
11 Feb 2010, 14:06

given, n is positive interger and n^2 - 1 = 8 * k + r -> r remainder what is r??

st 1) n is odd n^2-1 = (n+1) * (n-1) so n+1 and n-1 are consequetive even numbers... one of them will be multiple of 2 and the other will be multiple of 4. So n^2 - 1 will be evenly divided by 8 and r=0 Sufficient st 1) n is not divisible by 8. Not sufficient

Re: 700 Algrbra! Need help again. Thanks so much! [#permalink]
11 Feb 2010, 14:19

I like the approach in the post above best for this problem; when you see something like n^2 - 1 in a GMAT question, it will almost always be useful to use the difference of squares factorization: n^2 - 1 = (n+1)(n-1). A less elegant alternative is to write n = 2k + 1. Then n^2 - 1 = (2k + 1)^2 - 1 = 4k^2 + 4k + 1 - 1 = 4k^2 + 4k = 4(k)(k + 1), and since k and k+1 are consecutive integers, one of them must be divisible by 2, so 4(k)(k + 1) must be divisible by 4*2 = 8.

To answer the question in the original post, if n=1, then n^2 - 1 = 0. So the question becomes, what is the remainder when 0 is divided by 8? Well, 0 is divisible by every positive integer; the quotient is zero and the remainder is zero. If you think back to how you first learned division, this should hopefully be clear: if you have, say, 11 apples and 8 children, we can give each child 1 apple and we have 3 left over, so the quotient is 1 and the remainder is 3 when you divide eleven by eight. If we have 0 apples and 8 children, we can give each child 0 apples and we have 0 left over, so the quotient is 0 and the remainder is 0 when we divide zero by eight. _________________

Nov 2011: After years of development, I am now making my advanced Quant books and high-level problem sets available for sale. Contact me at ianstewartgmat at gmail.com for details.

Re: 700 Algrbra! Need help again. Thanks so much! [#permalink]
11 Feb 2010, 15:11

Thanks for the great help.

But I still feel very confused. The question is asking "what's the value of r"? I understand when n is 3,5,7, the r will be 1. However, if n is 1, r will be 0. In this case, we have two answers for r and we can't really tell the exact value for r, right? This is the reason why I don't think the first one work and the answer should be "E". Am I in the right path? Thanks for the help again.

Re: 700 Algrbra! Need help again. Thanks so much! [#permalink]
11 Feb 2010, 15:18

YTT wrote:

Thanks for the great help.

But I still feel very confused. The question is asking "what's the value of r"? I understand when n is 3,5,7, the r will be 1. However, if n is 1, r will be 0. In this case, we have two answers for r and we can't really tell the exact value for r, right? This is the reason why I don't think the first one work and the answer should be "E". Am I in the right path? Thanks for the help again.

No, the remainder will be zero for any of the values 1, 3, 5, or 7 (or for any other odd value of n). If, say, n=5, then n^2 - 1 = 25 - 1 = 24, and the remainder when we divide 24 by 8 is zero. _________________

Nov 2011: After years of development, I am now making my advanced Quant books and high-level problem sets available for sale. Contact me at ianstewartgmat at gmail.com for details.