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GRE Number Theory
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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.

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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:
23 and 1
1 is NOT a prime number.

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