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by candygal79 » Sat Mar 29, 2014 12:52 am
If x and y are prime numbers, is y(x-3) odd ?

1) x > 10

2) y < 3

My workout -

x can be 11 ,13 ,17 ,19 and so on forth
lets say x is 11

Y ( 11-3) = y (8) , it can't be odd since it is multiplying with 8

2 ) y < 3

y =2

HELP !
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Source: — Data Sufficiency |

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by [email protected] » Sat Mar 29, 2014 1:19 am
Hi candygal79,

This DS question is built on Number Properties.

We're told that X and Y are PRIME numbers. We're asked if "Y(X-3) is odd?" This is a YES/NO question.

Here are the Number Properties worth knowing for this question:
Odd x Odd = Odd
Even x Odd = Even
Even x Even = Even

Fact 1: X > 10

Since X is PRIME and X > 10, this means that X MUST be ODD. Furthermore, this means that (X-3) MUST be EVEN. Notice that ANY integer multiplied by an EVEN yields an EVEN? So Y(X-3) will NEVER be ODD. This is CONSISTENT.
Fact 1 is SUFFICIENT

Fact 2: Y < 3

Since Y is PRIME, it Y MUST be 2. Here, we're dealing with the SAME Number Property rule as in Fact 1: Since Y is 2, 2(X-3) MUST be EVEN, so it will NEVER be odd. This is CONSISTENT.
Fact 2 is SUFFICIENT

Final Answer: D

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by Brent@GMATPrepNow » Sat Mar 29, 2014 7:38 am
Rich's solution is great, so I won't say anything more about that.

I will say that GMAT questions featuring PRIME numbers are often testing your ability to recognize that not all prime numbers are odd.
There are many people who assume that 2 is not prime since it's even. However, an integer is prime if it has exactly 2 positive divisors. The positive divisors of 2 are 1 and 2, so 2 must, indeed, be prime. So, in reality, all prime numbers are odd with the exception of 2, which is even.

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by j_shreyans » Tue Feb 10, 2015 8:48 am
Hi All ,

Statement 2 says that y<3 .

so y can be negative also like (-2,-5......)

Please advise.

Thanks

Shreyans
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by GMATGuruNY » Tue Feb 10, 2015 8:54 am
j_shreyans wrote:Hi All ,

Statement 2 says that y<3 .

so y can be negative also like (-2,-5......)

Please advise.

Thanks

Shreyans
The prompt states that x and y are PRIME.
The smallest prime number is 2.
No negative integers are prime.
In statement 2, y must be a prime number less than 3.
Since 2 is the only prime number less than 3, statement 2 implies that y=2.
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