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what is the value of R

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what is the value of R

by gmatgmat » Sun Jul 13, 2008 2:05 am
if N is the +ve integer and r is the remainder when (n-1)(n+1) is divided by 24. What is the value of R?

1. N is divisible by 2.
2. N is divisible by 3.


Ans C ..

i dont know how i can find the number R.

Please help.

Thanks.
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Source: — Data Sufficiency |
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by anubhavmehrotra » Sun Jul 13, 2008 4:21 am
According to me.. answer should be E....

As:
1) divisible by 2.... number can be 2, 4, 6...
--> Taking 2... (2-1)(2+1)/24 ... remainder = 3 (this is your R)
--> Taking 4 ... (4-1)(4+1)/24 .. remainder = 15 .... Hence narrow choice to BCE...

2) divisible by 3 ... number can be 3, 6, 9 ...
--> Taking 3 .. 2*4 / 24 ... remainder = 8
--> Taking 6 .. 5*7 / 24 .. 35/24 .. remainder = 11 .. Hence as R is different .. eleminate B

3) divisible by 2 and 3 ... N can be 6, 12, 18 ...
--> Taking 6 .. remainder = 11
--> Taking 12 .. 11*13/24 = 143/24 .. remainder = 23 .. hence as R differs, we cannot say what R is... hence E ...

What say people ?
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by olika » Mon Jul 14, 2008 8:26 am
I've got E, too

We know that (n-1)(n+1)=24*x+R. We can write it in other way: (n*(n-1)+(n-1))/24=x+R/24

If we combine (1) and (2), we will get that n is a multiple of 6.
if n=6, then (6*5+5)/24=x+R/24, R=11
if n=12, then (12*11+11)/24=x+R/24, R=23

So, it can't be the C answer :shock:

Where did you get this question from?
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