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How many positive prime numbers are less than the integer n?

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How many positive prime numbers are less than the integer n? [#permalink] New post 26 Dec 2012, 23:50
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  25% (medium)

Question Stats:

68% (01:40) correct 32% (00:27) wrong based on 41 sessions
How many positive prime numbers are less than the integer n?

(1) 14 < n < 20
(2) 13 < n < 17
[Reveal] Spoiler: OA
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Re: how many positive prime numbers are less than the integer n? [#permalink] New post 27 Dec 2012, 00:16
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jsphcal wrote:
How many positive prime numbers are less than the integer n?



(1) 14 < n < 20


(2) 13 < n < 17


Lets consider all the positive prime numbers less than 20:

2, 3, 5, 7, 11, 13, 17, 19

(1) 14<n<20 and n is an integer

Value of n |||||||| positive prime numbers less than n
15 |||||||| 6 (2, 3, 5, 7, 11, 13)
16
17
18 |||||||| 7 (2, 3, 5, 7, 11, 13, 17)
19

Since we get 2 different answers, (1) is not sufficient

(2) 13 < n < 17


Value of n |||||||| positive prime numbers less than n
14 |||||||| 6 (2, 3, 5, 7, 11, 13)
15 |||||||| 6 (2, 3, 5, 7, 11, 13)
16 |||||||| 6 (2, 3, 5, 7, 11, 13)

Since (2) gives a definitive answer that there are 6 positive prime numbers less than n (whether n = 14, 15, or 16), (2) is sufficient.

The answer is B.
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Re: how many positive prime numbers are less than the integer n? [#permalink] New post 27 Dec 2012, 13:01
Edvento,

thanks this helps me to better understand why we ruled out 1 and why 2 is correct
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Re: How many positive prime numbers are less than the integer n? [#permalink] New post 28 Dec 2012, 03:23
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Re: How many positive prime numbers are less than the integer n? [#permalink] New post 30 Sep 2014, 09:41
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Re: How many positive prime numbers are less than the integer n?   [#permalink] 30 Sep 2014, 09:41
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