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

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

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

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.

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

Hi Bunuel,

Is there an alternative method to this where we do not have to plug in values at random?

Best-
Amit

For this particular question you have to test values. There is no purely algebraic way.

\sqrt{}

(1) 3x + 1 is prime

Let x =2........3 + 1 =7........Answer is Yes

Let x =6........18 + 1 =19........Answer is No

Insufficient

(2) 5x + 1 is prime


Let x =2........10+ 1 =11........Answer is Yes

Let x =6........30 + 1 =31........Answer is No

Insufficient

Combine 1 & 2

Use same examples..........No certain answer.

Insufficient

Answer: E

Bunuel chetan2u
can u clear this confusion:
3x+1 is prime
so 3x+1=6k+1 or 3x+1=6k-1
x=2k ----(A) or x=2k-(2/3)-----(B)
eqn b can never be prime as it will always be decimal
so answer is no
eqn A can be prime if k=1 or cannot be prime if k=2
so cannot say
My question is
will the answer be common to both A and B which is No
or answer will be
if i prove in any case x can be prime then it will be yes
else no
so we are considering its insufficent
can u please claer this ?
how to interpret answer from both the statemnts
Say it had been and case
then answer will be No ,right?

This is an excellent question on the properties of Primes. I’d also use the Units digit concept to solve the question since there is a multiple of 5 mentioned in one of the statements.

‘x’ is a positive integer. This means that x can take any value from the set {1,2,3,4,5……}.

From statement I, we know that 3x + 1 is prime. Since x is a positive integer, we can say that the smallest value we can take for 3x is 3, which is to say that the smallest value for x is 1. Therefore, 3x + 1 cannot be equal to 2 or 3.

If 3x + 1 = 5, 3x = 4. x is not an integer here. If 3x + 1 = 7, 3x = 6 which gives us x = 2. This value of x answers the main question with a YES since 2 is prime.

Any prime number greater than 3 can be written as 6k – 1 or 6k + 1 where k is a positive integer. A careful analysis will tell us that when we equate 3x + 1 to 6k + 1, we get x to be an integer. That’s why we got x = 2 in the example above.

The next two prime numbers that can be written in the form of 6k + 1 are 13 and 19. If 3x + 1 = 13, x = 4; if 3x + 1 = 19, x = 6. 4 and 6 are not prime numbers and hence answer the main question with a NO.

Statement I is insufficient. Answer options A and D can be eliminated. Possible answer options are B, C or E.

From statement II alone, 5x + 1 is a prime. Again, the smallest prime of this form can be 11. Clearly we are dealing with odd primes. Therefore, 5x has to be even. In other words, 5x is a multiple of 5 with units digit ZERO and so 5x + 1 is a prime with units digit 1. This means that x can be 2 or 6 or 8. We again face a YES NO situation.

Statement II alone is insufficient. Answer option B can be eliminated. Possible answer options are C or E.

Combining the data given in the two statements, we have x = 2 and x = 6 satisfying both statements at the same time. The combination of statements is insufficient as well.

The correct answer option is E.

Hope that helps!

VeritasKarishma

Ideally I would test numbers for this but if I were in a real hurry to solve this question, could I argue that prime numbers do not follow any regular pattern and therefore mark E straightaway?

Yes, you can but keep in mind that though there is no pattern to the generation of prime numbers, there are some properties that all prime numbers follow. That is, there is no property which tells you which number would be the next prime but once you find a prime, it will follow certain properties e.g. all primes greater than 3 are of the form (6x+1) or (6x-1).

Forget the conventional way to solve DS questions.

We will solve this DS question using the variable approach.

Remember the relation between the Variable Approach, and Common Mistake Types 3 and 4 (A and B)[Watch lessons on our website to master these approaches and tips]

Step 1: Apply Variable Approach(VA)

Step II: After applying VA, if C is the answer, check whether the question is key questions.

StepIII: If the question is not a key question, choose C as the probable answer, but if the question is a key question, apply CMT 3 and 4 (A or B).

Step IV: If CMT3 or 4 (A or B) is applied, choose either A, B, or D.

Let's apply CMT (2), which says there should be only one answer for the condition to be sufficient. Also, this is an integer question and, therefore, we will have to apply CMT 3 and 4 (A or B).

To master the Variable Approach, visit https://www.mathrevolution.com and check our lessons and proven techniques to score high in DS questions.

Let’s apply the 3 steps suggested previously. [Watch lessons on our website to master these 3 steps]

Step 1 of the Variable Approach: Modifying and rechecking the original condition and the question.

We have to find whether 'x' is prime.


Second and the third step of Variable Approach: From the original condition, we have 1 variable (x ). To match the number of variables with the number of equations, we need 1 equation. Since conditions (1) and (2) will provide 1 equation each, D would most likely be the answer.

But we know that this is a key question [Integer question] and if we get an easy C as an answer, we will choose A or B.

Let’s take a look at each condition.

Condition(1) tells us that 3x + 1 is prime.

=> If 3x + 1 = 7
=> x = 2 [prime]

=> But if 3x + 1 = 37
=> x = 12 [not prime]

Since the answer is not a unique YES or NO , the condition is not sufficient by CMT 1.


Condition(2) tells us that '5x + 1' is a prime.

=> If 5x + 1 = 11
=> x = 2 [prime]

=> But if 5x + 1 = 41
=> x = 8 [not prime]

Since the answer is not a unique YES or NO , the condition is not sufficient by CMT 1.

Combining equation '1' and '2' would also yield same YEs or NO as an answers.

Both conditions combined together are not sufficient.

So, E is the correct answer.

Answer: 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.

Answer: Option E

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Although the answer here is correct, when taken together x = 8 doesn't satisfy the first statement.

3(8) + 1 = NOT prime.

There is but one integer can both be prime number and be even number: 2. All other prime numbers are odd numbers.

Given that X is a positive integer, if 3X+1 or 5X+1 is a prime number, it must be an odd number as well.
If 3X+1 or 5X+1 is an odd number, then 3X or 5X must be an even number. Thus, X is an even number.

Here's the conclusion: even number X could be a prime number—2, or be a non-prime number, such as 4, 6, 8 etc.

As a mathematician, this is how I solved the problem:
There are infinite prime numbers,
There are infinite prime numbers of each type (we hardly have limiting results when it comes to primes.)
There are infinite primes of the type 3k+1, so are of the type 5m+1...are there more than 1 of the type belonging to both? Chances are high and also, I can't prove this in the exam..so i will go with Yes.

Posted from my mobile device

Hi,
Can you please share more qs for which testing values is an efficient method? Want to practice testing values more systematically :/
TIA!