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sumittaneja009: Senior | Next Rank: 100 Posts; Posts: 38; Joined: Sat Nov 08, 2008 4:36 am

by sumittaneja009 » Mon Jan 19, 2009 12:07 am

If n is a positive integer and r is the remainder when (n-1)(n+1) is divided by 24, what is the value of r?
1). n is not divisible by 2
2). n is not divisible by 3

Please explain?

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Source: Beat The GMAT — Data Sufficiency | Mon Jan 19, 2009 12:07 am

logitech: Legendary Member; Posts: 2134; Joined: Mon Oct 20, 2008 11:26 pmFollowed by: 25 members; Thanked: 237 times; GMAT Score:730

Re: GMAT Prep.

by logitech » Mon Jan 19, 2009 12:14 am

If n is a positive integer and r is the remainder when (n-1)(n+1) is divided by 24, what is the value of r?

1). n is not divisible by 2

So n is an odd number

n=1 - 0
n=3 - 8
n=5 - 0

INSUF ( I did not catch the n=3 case before )

2). n is not divisible by 3

n=1 - 0

n=2 - 3

Insuf

Together we know that n can not be 3 and n will be an ODD number

1, 5, 7, 11 and for all of these number the remainder will be 0

Choose C

Last edited by logitech on Mon Jan 19, 2009 9:52 am, edited 1 time in total.

LGTCH
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"DON'T LET ANYONE STEAL YOUR DREAM!"

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yalanand: Senior | Next Rank: 100 Posts; Posts: 79; Joined: Wed Feb 06, 2008 7:52 am; Thanked: 2 times; GMAT Score:620

by yalanand » Mon Jan 19, 2009 9:31 am

Logitech could you please explain me about this ??

1). n is not divisible by 2

So n is an odd number

n=1 - 0
n=3 - 0
n=5 - 0

So We will always get 0 as a remainder SUF

How is this sufficient ??

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logitech: Legendary Member; Posts: 2134; Joined: Mon Oct 20, 2008 11:26 pmFollowed by: 25 members; Thanked: 237 times; GMAT Score:730

by logitech » Mon Jan 19, 2009 9:49 am

yalanand wrote:Logitech could you please explain me about this ??

1). n is not divisible by 2

So n is an odd number

n=1 - 0
n=3 - 0
n=5 - 0

So We will always get 0 as a remainder SUF

How is this sufficient ??

If n is a positive integer and r is the remainder when (n-1)(n+1) is divided by 24, what is the value of r?

So we are asked to find the remainder. And in the first statement we all have 0 as the remainder when the number is divided by 24.

For example n=5

(n-1)(n+1) = 4x6=24 divided by 24 , remainder is 0

Now looking at my previous post, I realized that the only ODD number that will leave a different remainder is n=3

2x4=8 and when is divided by 24 , the remainder is 8

So it seems like A can not be sufficient because of the n=3 case.

Let's say what other will have to say :roll:

LGTCH
---------------------
"DON'T LET ANYONE STEAL YOUR DREAM!"

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