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# S97-10

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Math Expert
Joined: 02 Sep 2009
Posts: 41892

Kudos [?]: 129029 [0], given: 12187

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16 Sep 2014, 01:51
Expert's post
3
This post was
BOOKMARKED
00:00

Difficulty:

95% (hard)

Question Stats:

39% (01:21) correct 61% (01:25) wrong based on 77 sessions

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If $$n$$ is a positive integer greater than 1, then $$p(n)$$ represents the product of all the prime numbers less than or equal to $$n$$. The second smallest prime factor of $$p(12) + 11$$ is

A. 2
B. 11
C. 13
D. 17
E. 211
[Reveal] Spoiler: OA

_________________

Kudos [?]: 129029 [0], given: 12187

Math Expert
Joined: 02 Sep 2009
Posts: 41892

Kudos [?]: 129029 [0], given: 12187

### Show Tags

16 Sep 2014, 01:51
Expert's post
1
This post was
BOOKMARKED
Official Solution:

If $$n$$ is a positive integer greater than 1, then $$p(n)$$ represents the product of all the prime numbers less than or equal to $$n$$. The second smallest prime factor of $$p(12) + 11$$ is

A. 2
B. 11
C. 13
D. 17
E. 211

The quantity $$p(12)$$ equals the product of all the primes less than or equal to 12. Thus, the number we are looking for is this:
$$2 \times 3 \times 5 \times 7 \times 11 + 11$$

$$= 11 \times (2 \times 3 \times 5 \times 7+1)$$ [factor out the 11]

$$= 11 \times (210 + 1)$$ [do the arithmetic]
$$= 11 \times 211$$
$$= \text{some large number}$$

We want to keep this number factored, and in fact we need to find its prime factorization. So, we ask: is 211 prime? Well, 211 cannot be divided evenly by 2, 3, 5, or 7, because 211 equals a multiple of all those numbers (210), plus 1. The "plus 1" means that 211 won't be divisible by any of the same factors as 210 (except for 1). Is 211 divisible by 11 or by 13? No, as we can check quickly by long division.

And we only need to check possible prime factors up to the square root of 211 (which is between 14, the square root of 196, and 15, the square root of 225). If there is a pair of factors besides 211 and 1, one of the factors in the pair must be lower than the square root of 211 (and the other factor in the pair would be larger). Since there are no prime factors of 211 less than 14, we know that 211 is prime, and the second smallest prime factor of $$p(12) + 11 = 11 \times 211$$ is 211.

_________________

Kudos [?]: 129029 [0], given: 12187

Intern
Joined: 15 Jul 2014
Posts: 10

Kudos [?]: 2 [0], given: 25

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24 Nov 2014, 19:20
Can someone please explain what does the question mean by second smallest prime factor and why 11 is not the right answer?

Kudos [?]: 2 [0], given: 25

Math Expert
Joined: 02 Sep 2009
Posts: 41892

Kudos [?]: 129029 [1], given: 12187

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25 Nov 2014, 05:02
1
KUDOS
Expert's post
kritiu wrote:
Can someone please explain what does the question mean by second smallest prime factor and why 11 is not the right answer?

Because 11 is the smallest prime. The second smallest prime is 211.

If n is a positive integer greater than 1, then p(n) represents the product of all the prime numbers less than or equal to n. The second smallest prime factor of p(12)+11 is

A. 2
B. 11
C. 13
D. 17
E. 211

$$p(12)+11=2*3*5*7*11+11=11(2*3*5*7+1)=11*211$$. Both 11 and 211 are primes: 11 is the smallest prime of p(12)+11 and 211 is the second smallest prime of p(12)+11.

_________________

Kudos [?]: 129029 [1], given: 12187

Intern
Joined: 15 Jul 2014
Posts: 10

Kudos [?]: 2 [0], given: 25

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25 Nov 2014, 11:13
thanks Banuell. That helps!

Kudos [?]: 2 [0], given: 25

Intern
Joined: 17 Jul 2016
Posts: 10

Kudos [?]: 5 [0], given: 3

Location: India

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06 Feb 2017, 03:27
Hi

Why are we not considering 1 as one of the factors, then surely 11 would be the 2nd smallest?
Please correct me if i'm wrong.

Thanks
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Regards,
Lumin

Kudos [?]: 5 [0], given: 3

Math Expert
Joined: 02 Sep 2009
Posts: 41892

Kudos [?]: 129029 [0], given: 12187

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06 Feb 2017, 07:37
Ashandilya wrote:
Hi

Why are we not considering 1 as one of the factors, then surely 11 would be the 2nd smallest?
Please correct me if i'm wrong.

Thanks

Because 1 is not a prime number.
_________________

Kudos [?]: 129029 [0], given: 12187

VP
Joined: 14 Nov 2016
Posts: 1161

Kudos [?]: 1182 [0], given: 415

Location: Malaysia

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13 Feb 2017, 20:23
Bunuel wrote:
Official Solution:

If $$n$$ is a positive integer greater than 1, then $$p(n)$$ represents the product of all the prime numbers less than or equal to $$n$$. The second smallest prime factor of $$p(12) + 11$$ is

A. 2
B. 11
C. 13
D. 17
E. 211

The quantity $$p(12)$$ equals the product of all the primes less than or equal to 12. Thus, the number we are looking for is this:
$$2 \times 3 \times 5 \times 7 \times 11 + 11$$

$$= 11 \times (2 \times 3 \times 5 \times 7+1)$$ [factor out the 11]

$$= 11 \times (210 + 1)$$ [do the arithmetic]
$$= 11 \times 211$$
$$= \text{some large number}$$

We want to keep this number factored, and in fact we need to find its prime factorization. So, we ask: is 211 prime? Well, 211 cannot be divided evenly by 2, 3, 5, or 7, because 211 equals a multiple of all those numbers (210), plus 1. The "plus 1" means that 211 won't be divisible by any of the same factors as 210 (except for 1). Is 211 divisible by 11 or by 13? No, as we can check quickly by long division.

And we only need to check possible prime factors up to the square root of 211 (which is between 14, the square root of 196, and 15, the square root of 225).If there is a pair of factors besides 211 and 1, one of the factors in the pair must be lower than the square root of 211 (and the other factor in the pair would be larger). Since there are no prime factors of 211 less than 14, we know that 211 is prime, and the second smallest prime factor of $$p(12) + 11 = 11 \times 211$$ is 211.

Dear Bunuel, Do you have any example to illustrated the yellow highlight because I have no idea about what the statement is trying to convey.
_________________

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Kudos [?]: 1182 [0], given: 415

Re: S97-10   [#permalink] 13 Feb 2017, 20:23
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# S97-10

Moderator: Bunuel

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