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 divided by 8
OA A
Explanations please
GMAT prep Question-1
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n^2-1 = (n-1) (n+1)
Stmt I
n is odd which tell us n-1 and n+1 are both consecutive evens. The product of any 2 consecutive evens will always be divisible by 8
Remainder:0
SUFF
Stmt II
n is not divisible by 8
We can pick many n values with different remainders when divided by 8
INSUFF
Choose A
Stmt I
n is odd which tell us n-1 and n+1 are both consecutive evens. The product of any 2 consecutive evens will always be divisible by 8
Remainder:0
SUFF
Stmt II
n is not divisible by 8
We can pick many n values with different remainders when divided by 8
INSUFF
Choose A
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The method that I applied was plugging numbers
n>0 and an integer
1) N is odd
n^2-1; substituting n=3,5,7,9,11 all leads to remainder 0. As it is perfectly divisible by 8
1 is sufficient
2) N is not divisble by 8.
Lets substitute n=4,6,7
n=4; 15/8 remainder 7
n= 6; 35/8 remainder 3
n= 7; 48/8 remainder 0
Inconclusive results: not sufficient
Thus answer is A
n>0 and an integer
1) N is odd
n^2-1; substituting n=3,5,7,9,11 all leads to remainder 0. As it is perfectly divisible by 8
1 is sufficient
2) N is not divisble by 8.
Lets substitute n=4,6,7
n=4; 15/8 remainder 7
n= 6; 35/8 remainder 3
n= 7; 48/8 remainder 0
Inconclusive results: not sufficient
Thus answer is A
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BlackArrow,
Picking numbers for remainder problems is a sure shot approach and I too am a big fan of it.
Like u did we must always go with the approach that immediately comes to mind rather than trying to think of a text book solution if one does not come to mind.
Picking numbers works good with remainder probs i.e as soon as u start seeing a pattern for eg. in stament I where the remainder is always 0 when u pick a odd. Most probs can be solved this way.
Just my 2 cents in the mix...
Good luck.
Regards,
CR
Picking numbers for remainder problems is a sure shot approach and I too am a big fan of it.
Like u did we must always go with the approach that immediately comes to mind rather than trying to think of a text book solution if one does not come to mind.
Picking numbers works good with remainder probs i.e as soon as u start seeing a pattern for eg. in stament I where the remainder is always 0 when u pick a odd. Most probs can be solved this way.
Just my 2 cents in the mix...
Good luck.
Regards,
CR
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another soln.
the problem can be written as
n^2 -1 = 8k + r and we have find r.
stmt 1: n is odd.
We know square of n is always odd if n is odd.
so n^2 -1 is even.
Now choose values for n.
1, 3, 5, 7.
(n^2 -1) is 0, 8, 24, 48.
In every case the remainder is 0 when divided by 8.
So suffiient.
Stmt 2:
n is not divisible by 8.
So the remainder can be any number 1 - 7.
So insufficient.
Ans is A.
HT helps
the problem can be written as
n^2 -1 = 8k + r and we have find r.
stmt 1: n is odd.
We know square of n is always odd if n is odd.
so n^2 -1 is even.
Now choose values for n.
1, 3, 5, 7.
(n^2 -1) is 0, 8, 24, 48.
In every case the remainder is 0 when divided by 8.
So suffiient.
Stmt 2:
n is not divisible by 8.
So the remainder can be any number 1 - 7.
So insufficient.
Ans is A.
HT helps
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Cramya,
Can you please let me know how to take the screenshots from the GMAT prep test??
I gave my GMAT prep yesterday and did exceptionally bad, and now I want to discuss some questions.
But I am not able to get the screenshots,some questions require figures and it will be otherwise very time consuming for me to post those questions.
Regards,
Karan
Can you please let me know how to take the screenshots from the GMAT prep test??
I gave my GMAT prep yesterday and did exceptionally bad, and now I want to discuss some questions.
But I am not able to get the screenshots,some questions require figures and it will be otherwise very time consuming for me to post those questions.
Regards,
Karan
- Vemuri
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I think that should be easy. Just go to the review page, open up the question & use the "Print Scrn" button on your keyboard to take a screen shot of the page. Crop the image as desired, save it as an image & upload in this forum. If you are doing the above & still not able to copy the screen shot, then you are probably having a memory issue. Reboot your machine & try again.gmat740 wrote:Cramya,
Can you please let me know how to take the screenshots from the GMAT prep test??
I gave my GMAT prep yesterday and did exceptionally bad, and now I want to discuss some questions.
But I am not able to get the screenshots,some questions require figures and it will be otherwise very time consuming for me to post those questions.
Regards,
Karan