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Re: Is the positive integer y a prime number? [#permalink]

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31 Aug 2016, 07:30

1

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Stat 1: 80 < y < 95, We get both prime and nonprime numbers =Insufficient.

Stat 2: y= 3x + 1, the result must be multiple of y, and x is positive integer, X and y are unknown =Insufficient.

both statements the result must be multiple of y , and x is positive integer, possible values of y for x to be integer are 82,85,88,91,94, all are nonprimes= sufficient condition to answer the question,

(1) 80 < y < 95 (2) y = 3x + 1, where x is a positive integer

Target question:Is y a prime number?

Given: y is a positive integer

Statement 1: 80 < y < 95 This statement doesn't FEEL sufficient, so I'll TEST some values. There are several values of y that satisfy statement 1. Here are two: Case a: y = 81, in which case y is NOT prime Case b: y = 83, in which case y IS prime Since we cannot answer the target question with certainty, statement 1 is NOT SUFFICIENT

Statement 2: y = 3x + 1, where x is a positive integer There are several values of y that satisfy statement 2. Here are two: Case a: y = (3)(1) + 1 = 4, in which case y is NOT prime Case b: y = (3)(2) + 1 = 7, in which case y IS prime Since we cannot answer the target question with certainty, statement 2 is NOT SUFFICIENT

Statements 1 and 2 combined Let's list ALL possible values of y. y = (3)(27) + 1 = 82 (82 is NOT prime) y = (3)(28) + 1 = 85 (85 is NOT prime) y = (3)(29) + 1 = 88 (88 is NOT prime) y = (3)(30) + 1 = 91 (91 is NOT prime)(91 = 13 times 7) y = (3)(31) + 1 = 94 (94 is NOT prime) So, we can conclude that y is NOT prime Since we can answer the target question with certainty, the combined statements are SUFFICIENT

Re: Is the positive integer y a prime number? [#permalink]

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01 Sep 2016, 19:16

Answer should be both statements together are sufficient to get the answer . From first statement you are getting many prime numbers not a definite value , but if you use y=3x+1 then the only prime number between 80 and 95 is 91 when X=30 which will give you 91 , hence addresses first statement as well

Re: Is the positive integer y a prime number? [#permalink]

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01 Sep 2016, 23:15

vikasku3087 wrote:

Answer should be both statements together are sufficient to get the answer . From first statement you are getting many prime numbers not a definite value , but if you use y=3x+1 then the only prime number between 80 and 95 is 91 when X=30 which will give you 91 , hence addresses first statement as well

That is incorrect mate. If you use both statements together, it proves none of the numbers are prime. If you get even one number as prime, you're getting 2 answers hence, insufficient.

In this case, all the numbers are not prime, and that's why the 1) & 2) are sufficient together and C is the answer. Be careful with this when solving DS questions.

Re: Is the positive integer y a prime number? [#permalink]

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02 Sep 2016, 03:59

Question has a question mark - it is asking that whether y is prime number or not? So not is also an answer . It is simple case of putting number in second equation and testing -

(1) 80 < y < 95 (2) y = 3x + 1, where x is a positive integer

Anybody please tell how C is the correct answer. As per me, E is the correct answer

To demonstrate that the correct answer is E, you must show that there are two possible values for y that satisfy BOTH statements AND yield conflicting answer choices to the target question (is y prime?). When we combine the statements, we can see that y can equal 82 or 85 or 88 or 91 or 94 In all 5 possible cases, the answer to the target question is the same; y is not prime. In other words, we can be CERTAIN that the answer to the target question is NO - y is not prime.

Re: Is the positive integer y a prime number? [#permalink]

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06 Oct 2017, 09:00

Bunuel wrote:

Is the positive integer y a prime number?

(1) 80 < y < 95 (2) y = 3x + 1, where x is a positive integer

each alone is obviously insuff

both together

the min value of x in y = 3x+1 is 27.... to satisfy 80<y<95 , all primes >3 are in the form 6n+1 or 6n-1 , i.e. before or after a multiple of 6

in the given range multiples of 6 are 84 , 90 , check around 84 and 90 ( 83,85) and (89,91) ... of which only 85 and 91 satisfy the form (3x+1)both of which are non prime .... suff

C

gmatclubot

Re: Is the positive integer y a prime number?
[#permalink]
06 Oct 2017, 09:00