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C
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Difficulty:
45%
(medium)
Question Stats:
61% (01:35) correct
39%
(01:41)
wrong
based on 76
sessions
History
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Time
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Not Attempted Yet
If n is an integer, is n-2 a prime number?
(1) n - 1 is a prime number
(2) n is odd
Source: Jeff Sackmann
(1) n - 1 is a prime number
(2) n is odd
Source: Jeff Sackmann
I'm trying to solve the above question, but can't understand why we are getting C as the official answer. For St 1) if N = 3, then 3-1 = 2 (which is prime). However, then 3-2 = 1 (which is not prime). If N = 4, then 4-1 = 3 (prime), and 4-2 = 2 (prime). Since we have two answers, therefore this statement is not sufficient.
For St 2) if N is odd, we again two different answers if N = 3 or N = 5. This means this statement is not sufficient.
If we combine the statements together, and again use N = 3 or 5 (both odd numbers), then we still get two different answers:
If N = 3, then 3-1 = 2 (prime) and 3-2 = 1 (not prime)
If N = 5, then 5-1 = 4 (not prime) and 5-2 = 3 (prime)
Thus, shouldn't the answer be E? Can someone please clarify? Thanks!
For St 2) if N is odd, we again two different answers if N = 3 or N = 5. This means this statement is not sufficient.
If we combine the statements together, and again use N = 3 or 5 (both odd numbers), then we still get two different answers:
If N = 3, then 3-1 = 2 (prime) and 3-2 = 1 (not prime)
If N = 5, then 5-1 = 4 (not prime) and 5-2 = 3 (prime)
Thus, shouldn't the answer be E? Can someone please clarify? Thanks!
25 Jul 2019, 23:32
Kudos
Bookmarks
FJ24
If n is an integer, is n-2 a prime number?
(1) n - 1 is a prime number
(2) n is odd
Source: Jeff Sackmann
(1) n - 1 is a prime number
(2) n is odd
Source: Jeff Sackmann
I'm trying to solve the above question, but can't understand why we are getting C as the official answer. For St 1) if N = 3, then 3-1 = 2 (which is prime). However, then 3-2 = 1 (which is not prime). If N = 4, then 4-1 = 3 (prime), and 4-2 = 2 (prime). Since we have two answers, therefore this statement is not sufficient.
For St 2) if N is odd, we again two different answers if N = 3 or N = 5. This means this statement is not sufficient.
If we combine the statements together, and again use N = 3 or 5 (both odd numbers), then we still get two different answers:
If N = 3, then 3-1 = 2 (prime) and 3-2 = 1 (not prime)
If N = 5, then 5-1 = 4 (not prime) and 5-2 = 3 (prime)
Thus, shouldn't the answer be E? Can someone please clarify? Thanks!
For St 2) if N is odd, we again two different answers if N = 3 or N = 5. This means this statement is not sufficient.
If we combine the statements together, and again use N = 3 or 5 (both odd numbers), then we still get two different answers:
If N = 3, then 3-1 = 2 (prime) and 3-2 = 1 (not prime)
If N = 5, then 5-1 = 4 (not prime) and 5-2 = 3 (prime)
Thus, shouldn't the answer be E? Can someone please clarify? Thanks!
If n is an integer, is n-2 a prime number?
(1) n - 1 is a prime number.
Is it possible n - 2 to be a prime, if n - 1 is a prime? Yes. There is one pair of consecutive integers where both of them are primes: 2 and 3. So, if n - 1 = 3, then n - 2 = 2 = prime.
Is it possible n - 2 NOT to be a prime, if n - 1 is a prime? Certainly. For example, if n - 1 = 2, then n - 2 = 1, which is not prime.
Not sufficient.
(2) n is odd. Clearly insufficient. For example, consider n = 1 for a NO answer and n = 5 for an YES answer.
(1)+(2) Since from (2) n is odd, then from (1) n - 1 = odd - odd = even. There is only one even prime: 2. Thus, n - 1 = 2, so n - 2 = 1, which is not a prime. Sufficient.
Answer: C.
27 Jul 2019, 12:56
Kudos
Bookmarks
Bunuel
FJ24
If n is an integer, is n-2 a prime number?
(1) n - 1 is a prime number
(2) n is odd
Source: Jeff Sackmann
(1) n - 1 is a prime number
(2) n is odd
Source: Jeff Sackmann
I'm trying to solve the above question, but can't understand why we are getting C as the official answer. For St 1) if N = 3, then 3-1 = 2 (which is prime). However, then 3-2 = 1 (which is not prime). If N = 4, then 4-1 = 3 (prime), and 4-2 = 2 (prime). Since we have two answers, therefore this statement is not sufficient.
For St 2) if N is odd, we again two different answers if N = 3 or N = 5. This means this statement is not sufficient.
If we combine the statements together, and again use N = 3 or 5 (both odd numbers), then we still get two different answers:
If N = 3, then 3-1 = 2 (prime) and 3-2 = 1 (not prime)
If N = 5, then 5-1 = 4 (not prime) and 5-2 = 3 (prime)
Thus, shouldn't the answer be E? Can someone please clarify? Thanks!
For St 2) if N is odd, we again two different answers if N = 3 or N = 5. This means this statement is not sufficient.
If we combine the statements together, and again use N = 3 or 5 (both odd numbers), then we still get two different answers:
If N = 3, then 3-1 = 2 (prime) and 3-2 = 1 (not prime)
If N = 5, then 5-1 = 4 (not prime) and 5-2 = 3 (prime)
Thus, shouldn't the answer be E? Can someone please clarify? Thanks!
If n is an integer, is n-2 a prime number?
(1) n - 1 is a prime number.
Is it possible n - 2 to be a prime, if n - 1 is a prime? Yes. There is one pair of consecutive integers where both of them are primes: 2 and 3. So, if n - 1 = 3, then n - 2 = 2 = prime.
Is it possible n - 2 NOT to be a prime, if n - 1 is a prime? Certainly. For example, if n - 1 = 2, then n - 2 = 1, which is not prime.
Not sufficient.
(2) n is odd. Clearly insufficient. For example, consider n = 1 for a NO answer and n = 5 for an YES answer.
(1)+(2) Since from (2) n is odd, then from (1) n - 1 = odd - odd = even. There is only one even prime: 2. Thus, n - 1 = 2, so n - 2 = 1, which is not a prime. Sufficient.
Answer: C.
Thank you! This clears up my confusion.
