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# DS-Gmatprep Divisibility

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Senior Manager
Joined: 05 Jun 2005
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29 Sep 2006, 16:34
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This topic is locked. If you want to discuss this question please re-post it in the respective forum.

Please explain. I am trying to understand the concept of this consecutive interger stuff with (x+1)(x-1) and also what is the rule with n(n-1)(n+1). Seen that as well
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Intern
Joined: 24 Aug 2006
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30 Sep 2006, 07:09
Ok we know p^2 - n^2 = (p + n)(p - n)

We need to find the remainder when (p + n)(p - n) is divided by 15.

(1) p + n = 5x + 1 . Insuff
(2) p - n = 3y + 1 . Insuff

Combining we get (p + n)(p - n) = (5x + 1)(3y + 1)

= 15xy + 5x + 3y + 1 Hence Insuff

Another way is to pick number

From (1) we get > 1, 6 , 11 , 16 etc
From (2) we get > 1, 4 , 7 , 10 etc

Combining we get combinations such as 24, 77 , 6 which all have different remainders. Hence E

OA?

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Current Student
Joined: 29 Jan 2005
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30 Sep 2006, 07:19
Picking numbers takes way too long here. Better to FOIL and try to find a common solution between the statements. In this case there isn't one.

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Senior Manager
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30 Sep 2006, 07:23
Thanks JayP and Matt, the answer is E. I like the foil idea better here

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30 Sep 2006, 07:23
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