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]

27 Dec 2012, 00:50

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

(1) 14 < n < 20
(2) 13 < n < 17

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Re: how many positive prime numbers are less than the integer n? [#permalink]

27 Dec 2012, 01:16

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]

27 Dec 2012, 14:01

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]

28 Dec 2012, 04:23

jsphcal wrote:

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

(1) 14 < n < 20
(2) 13 < n < 17

Similar question to practice: collection-of-12-ds-questions-85441-60.html#p809394
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Re: How many positive prime numbers are less than the integer n? [#permalink] 28 Dec 2012, 04:23

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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?

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

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