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# If two of the four expressions

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If two of the four expressions [#permalink]  13 Nov 2008, 02:24
Help!
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product.JPG [ 39.82 KiB | Viewed 751 times ]

Manager
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Re: GMATPREP-product [#permalink]  13 Nov 2008, 02:45
x^2 - (by)^2 = (x-by)(x+by)

The probability P1 to get (x-by) = 1/4 as only x-y satisfies
Once taking x-y out of the list, the probability P2 to get x + by is 2 out of 3 (x+y and x+5y)

P = P1 * P2 = 1/4 * 2/3 = 1/6
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Re: GMATPREP-product [#permalink]  13 Nov 2008, 06:22
to satisfy the condition x^2-y^2 we need to have this form (x-y)(x+y)

so... two expressions are in the form (x-y) and two (x+y)

so 2*2/4*3*2*1=4/24=1/6

this is a tricky one...
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Re: GMATPREP-product [#permalink]  13 Nov 2008, 11:25
pretty straight forward

to get product of the nature of x^2 - (by)^2 we need to pick both (x+y) and (x-y) from a total of 4 expressions

probability= 2/4*1/3=1/6
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Re: GMATPREP-product [#permalink]  13 Nov 2008, 19:13
1/4C2 = 1/6 ( 1 for choosing x+y, x-y)
Re: GMATPREP-product   [#permalink] 13 Nov 2008, 19:13
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