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GRE Number Theory http://www.800score.com/forum/viewtopic.php?f=12&t=5976 
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Author:  questioner [ Thu Sep 13, 2012 4:18 am ] 
Post subject:  GRE Number Theory 
Which of the following integers is/are the product of exactly two different prime numbers? 21, 22, 23, 24, 25 Credited Answer(s): 21, 22 The correct answers are 21 and 22. Let's factor each possible answer. Choice A: 21 = 3 × 7 Choice B: 22 = 2 × 11. Choice C: 23 = prime number. Choice D: 24 = 2 × 2 × 3 × 3. Choice E: 25 = 5 × 5. Only the first two choices (21 and 22) are the product of exactly two different prime numbers.  23 is also the product of the two following different prime numbers: 23 and 1 
Author:  Gennadiy [ Thu Sep 13, 2012 4:18 am ] 
Post subject:  Re: GRE Number Theory 
Quote: 23 is also the product of the two following different prime numbers: 1 is NOT a prime number.
23 and 1 
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