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If x is a positive integer, then is x prime? 1) 3x + 1 is prime 2) 5x

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If x is a positive integer, then is x prime? 1) 3x + 1 is prime 2) 5x [#permalink]

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New post 16 Jun 2016, 05:03
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A
B
C
D
E

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  65% (hard)

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59% (01:24) correct 41% (01:23) wrong based on 874 sessions

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Re: If x is a positive integer, then is x prime? 1) 3x + 1 is prime 2) 5x [#permalink]

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New post 16 Jun 2016, 07:30
Bunuel wrote:
If x is a positive integer, then is x prime?

(1) 3x + 1 is prime
(2) 5x + 1 is prime

x = +ve integer
(1) 3x + 1 is prime
if 3x+1 = 7 = prime
=> x= 2 , which is prime

if 3x+1 = 19 = prime
=> x = 6 , which is not a prime
Not sufficient

(2) 5x + 1 is prime
if 5x + 1 = 11 = prime
=> x = 2 , which is prime

if 5x + 1 = 31 = prime
=> x = 6 , which is not a prime
Not sufficient

Combining 1 and 2 , we get

Still Not sufficient
Answer E
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Re: If x is a positive integer, then is x prime? 1) 3x + 1 is prime 2) 5x [#permalink]

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Bunuel wrote:
If x is a positive integer, then is x prime?

(1) 3x + 1 is prime
(2) 5x + 1 is prime


Statement 1:-

If x is 2, 4 , 6.... then 3x + 1 is prime

if x is 1, 3, 5.... then 3x + 1 is not prime

Not sufficient as x may or may not be prime.

Statement 2:-

If x is 1, 3, 5... then 5x + 1 is not prime

if x is 2, 6... then 5x + 1 is prime.

Not sufficient as x may or may not be prime.

Both statements combined together do not give a unique solution. Hence the answer is E
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Re: If x is a positive integer, then is x prime? 1) 3x + 1 is prime 2) 5x [#permalink]

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There is one variable (x) in the original condition. So we need 1 equation. Since the condition 1) and the condition 2) each has 1 equation, there is high chance that D is the correct answer.
The condition 1) has x=2 yes, x=6 no not suffi.
The condition 2) has x=2 yes, x=6 no not suffi.
Since the condition 1) and the condition 2) have x=2 yes, x=6 no not suffi, the correct answer is E.

- For cases where we need 1 more equation, such as original conditions with “1 variable”, or “2 variables and 1 equation”, or “3 variables and 2 equations”, we have 1 equation each in both 1) and 2). Therefore, there is 59 % chance that D is the answer, while A or B has 38% chance and C or E has 3% chance. Since D is most likely to be the answer using 1) and 2) separately according to DS definition. Obviously there may be cases where the answer is A, B, C or E.
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Re: If x is a positive integer, then is x prime? 1) 3x + 1 is prime 2) 5x [#permalink]

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New post 17 Jun 2016, 02:41
Bunuel wrote:
If x is a positive integer, then is x prime?

(1) 3x + 1 is prime
(2) 5x + 1 is prime


primes > 3 are always of the form 6k +/- 1

1) 3x + 1 = 6k+1
=> x = 6k/3 = 2k
2k can be prime (2) or not (4,6,8,...)
insufficient

2) 5x+1 = 6k+1
=> x = 6k/5
here x cannot be a prime

5x+1 = 6k-1
=> x = (6k-2)/5
if k=2, then x=2 (prime)
if k=7, then x=8 (not a prime)
insufficient

(1)+(2)
x=2 or x=8 (more common values might exist but i am not bothering to calculate since this is enough to answer)
insufficient.

Hence, E.
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If x is a positive integer, then is x prime? 1) 3x + 1 is prime 2) 5x [#permalink]

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A quick trick: plug in only even numbers in \(x\).
Remember that with the exception of 2, prime numbers are odd.

For the equation \(3x + 1\) and \(5x + 1\) to have an odd prime result, we need to plug in even numbers 2, 4, 6 to \(x\).

Because combining both A and B gives us \(x = 2\) (prime) and \(x = 6\) (not prime), our answer is E.
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Re: If x is a positive integer, then is x prime? 1) 3x + 1 is prime 2) 5x [#permalink]

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Bunuel wrote:
If x is a positive integer, then is x prime?

(1) 3x + 1 is prime
(2) 5x + 1 is prime


We are given that x is a positive integer and must determine whether x is prime.

Statement One Alone:

3x + 1 is prime.

Using the information in statement one, x does not necessarily have to be prime. For instance, if x = 2, then 3x + 1 = 7 is prime, or if x = 4, then 3x + 1 = 13 is prime. In the former case, x = 2 is prime; however, in the latter case, x = 4 is not prime. Statement one alone is not sufficient to answer the question. We can eliminate answer choices A and D.

Statement Two Alone:

5x + 1 is prime.

Using the information in statement two, x does not necessarily have to be prime. For instance, if x = 2, then 5x + 1 = 11 is prime, or if x = 6, then 5x + 1 = 31 is prime. In the former case, x = 2 is prime; however, in the latter case, x = 6 is not prime. Statement two alone is not sufficient to answer the question. We can eliminate answer choice B.

Statements One and Two Together:

Using the information from statements one and two, we still cannot determine whether x is prime. For instance, if x = 2, then both 3x +1 = 7 and 5x + 1 = 11 are prime, or if x = 12, then both 3x + 1 = 37 and 5x + 1 = 61 are prime. In the former case, x = 2 is prime; however, in the latter case, x = 12 is not prime.

Answer: E
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Re: If x is a positive integer, then is x prime? 1) 3x + 1 is prime 2) 5x [#permalink]

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New post 28 Mar 2017, 19:08
Plug in prime numbers as a result of the equation
1)
3x+1=7
x=2 (prime)

3x+1=19
x=6 (not prime); hence NS

2)
5x+1=11
x=2 (prime)

5x+1=31
x=6 (not prime) ; hence NS

1) & 2)
x=2 or x= 6 and both equations would still yield a prime number.

Correct answer E)
Re: If x is a positive integer, then is x prime? 1) 3x + 1 is prime 2) 5x   [#permalink] 28 Mar 2017, 19:08
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