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If two fo the four expressions x+y, x+5y, x-y, and 5x-y are

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Intern
Joined: 03 Mar 2009
Posts: 48
If two fo the four expressions x+y, x+5y, x-y, and 5x-y are [#permalink]

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12 Apr 2009, 21:20
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If two fo the four expressions x+y, x+5y, x-y, and 5x-y are chosen at random, what ist he probability that their product will be of the form x^2-(by)^2, where b is an integer?

1/2
1/3
1/4
1/5
1/6

I get 1/3 because there solutions can only be (x+y)(x-y) and (x+y)(x-5y), and there can only be 6 combinations.

OA is E: 1/6
Intern
Joined: 03 Mar 2009
Posts: 48
Re: DS: GMATPrep Test 2 (Four equations [#permalink]

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12 Apr 2009, 21:22
Crap, could a mod please move this to the PS forum? Sorry.
Manager
Joined: 14 Nov 2008
Posts: 195
Schools: Stanford...Wait, I will come!!!
Re: DS: GMATPrep Test 2 (Four equations [#permalink]

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12 Apr 2009, 23:57
Nach0 wrote:
If two fo the four expressions x+y, x+5y, x-y, and 5x-y are chosen at random, what ist he probability that their product will be of the form x^2-(by)^2, where b is an integer?

1/2
1/3
1/4
1/5
1/6

I get 1/3 because there solutions can only be (x+y)(x-y) and (x+y)(x-5y), and there can only be 6 combinations.

OA is E: 1/6

[color=#FF0000](x+y)(x-5y)---> will give you x^2 -4xy - 5y^2 ... so it is not a solution.. only one solution is there which you have already mentioned.. the probablility will be 1/6 only.[/color]
Current Student
Joined: 13 Jan 2009
Posts: 364
Location: India
Re: DS: GMATPrep Test 2 (Four equations [#permalink]

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13 Apr 2009, 17:28
1/4c2= 1/6

Fav case (x-y)(x+Y)
Re: DS: GMATPrep Test 2 (Four equations   [#permalink] 13 Apr 2009, 17:28
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