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## GMATPREP Test #2 question

This topic has 1 member reply
MaybeMBA Newbie | Next Rank: 10 Posts
Joined
26 Dec 2006
Posted:
1 messages

#### GMATPREP Test #2 question

Tue Dec 26, 2006 11:44 am
Elapsed Time: 00:00
• Lap #[LAPCOUNT] ([LAPTIME])
Hi, this is my first post so hopefully I'm following the proper protocol.

This is not a particularly difficult question, so I'm alarmed that it's stumped me:

If two of the four expressions (x+y), (x+5y), (x-y), and (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?

(A) 1/2 (B) 1/3 (C) 1/4 (D) 1/5 (E) 1/6

I chose C, the right answer is E.

I assumed there was a 1/2 chance of getting an x+y form expression and a 1/2 chance of getting an x-y form expression. The joint chance of getting to independent events should be (1/2)(1/2) = 1/4. But I'm obviously overlooking something.

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thankont Senior | Next Rank: 100 Posts
Joined
15 Dec 2006
Posted:
41 messages
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Wed Dec 27, 2006 7:19 am
One approach could be this: since only 1 pair (x+y), (x-y) meets the requirements we have 1/6 (6= 4C2 --> choose 2 out of 4 = 6 pairs)

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