OG If x is a positive integer, then is x prime?

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

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

E

Given: x is a positive integer
To find out: is x prime?

Statement 1: 3x + 1 is prime

Since x is a positive integer, the minimum value of x = 1, thus the minimum value of 3x + 1 = 3*1 + 1 = 4, but '4' is non-prime, thus x = 1 is not a valid value.

Let's try with x = 2, then 3x = 1 = 3*2 + 1 = 7, a prime number. So, if x = 2, then x is prime.

We know that all the prime numbers except 2 are odd, thus, 3x + 1 = Odd.

3x + 1 = Odd
3x = Odd – 1 = Odd – Odd = Even
x = Even / 3 = Even / Odd = Even (since x is a positve integer);

Note that if an even integer is completely divisible by an odd integer, the quotient is even.

So, we have x = Even

If x = 2, the answer is Yes; however, if x ≠ 2, the answer is No. Insufficient.

Statement 2: 5x + 1 is prime

At x = 2, we see that 5x + 1 = 5*2 + 1 = 11, a prime number, so if x = 2, the answer is yes.

As discussed in Statement 1, we know that all the prime numbers except 2 are odd, thus, 5x + 1 = Odd.

5x + 1 = Odd
5x = Odd – 1 = Odd – Odd = Even
x = Even / 5 = Even / Odd = Even (since x is a positve integer);

So, we have x = Even

If x = 2, the answer is Yes; however, if x ≠ 2, the answer is No. Insufficient.

Statement 1 & 2:

x = 2, implying x is prime, and x = other even integers are applicable in both the statements, thus there is no unique answer. Insufficient.

The correct answer: E

Hope this helps!

Download free ebook: Manhattan Review GMAT Quantitative Question Bank Guide

-Jay
_________________
Manhattan Review GMAT Prep

Locations: New York | Barcelona | Manila | Melbourne | and many more...

Schedule your free consultation with an experienced GMAT Prep Advisor! Click here.