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

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


(1) between 14 to 20 we have 2 prime numbers if n=20 , if n= 18 one prime so the n value is unknow, i.e not sufficient.

(2) between 13 and 14 if n=14 no. of prime's will be 0 and , if = 15 no. of prime's will still be 0.
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Re: How many positive prime numbers are less than the integer n? [#permalink]
Edvento
jsphcal
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.

Hi Edvento, thanks for your explanation.
Not sure what is |||||||| mean?
Also why is prime number for St1 is not 17 & 19 but 15 & 18 ( these are not prime numbers)? Same with St2.
Could you help clarify? Thanks
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Re: How many positive prime numbers are less than the integer n? [#permalink]
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