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Probability

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Probability

by danjuma » Sun Oct 10, 2010 9:15 am
If 2 of the 4 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 square minus b( y square), where b is an integer?

a.1/2 b.1/3 c.1/6 d.1/5 e.1/4
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by this_time_i_will » Sun Oct 10, 2010 9:28 am
total number of possible outcome = 4c2 = 6
favorable outcome = 1. (selection of x+y and x-y)

so required probability = 1/6
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by NKB » Sun Oct 10, 2010 9:31 am
IMO right ans is C

only 2 pairs will give the required format i.e
1) X-Y & X +Y = X^2 - Y^2
2) X-5Y & X+5Y = X^2 -25Y^2

And in all there are 4 C 2 ways of selecting 2 pairs out of 4
Probability = 2/12 = 1/6
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