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Probability GMAT Prep.

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Probability GMAT Prep.

by taneja.niks » Wed Oct 20, 2010 10:27 am
If two of the four expressions x+y,x+5y,x-y,5x-y are chosen at random, What is the probability that their product will be of the form x^2- (by)^2, Where b is an integer

1)1/2
2)1/3
3)1/4
4)1/5
6)1/6
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Source: — Data Sufficiency |
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by shovan85 » Wed Oct 20, 2010 11:02 am
IMO E

See required form x^2- (by)^2, Where b is an integer .
As b is a integer the problem becomes simple :)

x+y,x+5y,x-y,5x-y these are total 4 options
for the product of 2 exp we need 2 options out of these 4

Hence, total number of possible selections = C(4,2) = 4!/(2!*2!) = 6

Out of the 4 Exp only x+y,x-y can give you the required form when b is an integer.
So number of selections getting us the required form is 1.

Thus probability = 1/6
If the problem is Easy Respect it, if the problem is tough Attack it
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