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If x is an integer, does x have a factor n such that 1 <

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If x is an integer, does x have a factor n such that 1 <

by rakeshd347 » Thu Oct 03, 2013 7:53 pm
If x is an integer, does x have a factor n such that 1 < n < x?

(1) x > 3!

(2) 15! + 2 ≤ x ≤ 15! + 15

OA is B

I knew that 1 is totally not sufficient os I was between B C and E.
Now They don't expect us to calculate this factorial So I guessed this B but what is the logic behind it. I was lucky to guess correct but want to understand the logic. I seem to understand the x part but I can't understand how we relate it N whether N is a factor of X or not.

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by GMATGuruNY » Thu Oct 03, 2013 8:05 pm
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by Brent@GMATPrepNow » Thu Oct 03, 2013 10:36 pm
rakeshd347 wrote:If x is an integer, does x have a factor n such that 1 < n < x?

(1) x > 3!

(2) 15! + 2 ≤ x ≤ 15! + 15

OA is B
This is a great candidate for rephrasing the target question.
What kinds of integers have a factor n such that 1 < n < x? Non-prime integers. So, we're really just asking whether or not n is a non-prime integer. Let's make it even easier on ourselves and ask . . .

Rephrased target question: Is x prime?

Statement 1: x > 3!
In other words, x > 6
case a) x = 7, in which case x is prime
case b) x = 8, in which case x is not prime
Statement 1 is NOT SUFFICIENT

Statement 2: 15! + 2 ≤ x ≤ 15! + 15
This is saying that x can have one of 14 different possible values. So, let's begin checking some values.

Is 15! + 2 prime? No.
Notice that 15! = (15)(14)(13)...(3)(2)(1)
So, we can factor a 2 out of 15! + 2, to get:
15! + 2 = 2[(15)(14)(13)...(3)(1) + 1]
This means that 2 is a factor of 15! + 2, which means it is not prime.

Next, 15! + 3 prime? No.
Notice that 15! = (15)(14)(13)...(4)(3)(2)(1)
So, we can factor a 3 out of 15! + 3, to get:
15! + 3 = 3[(15)(14)(13)...(4)(2)(1) + 1]
This means that 3 is a factor of 15! + 3, which means it is not prime.

We can continue this process to show that none of the 14 possible values of x are prime.
As such, statement 2 is SUFFICIENT and the answer is B

Cheers,
Brent
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by ceilidh.erickson » Fri Oct 04, 2013 12:59 pm
These explanations are great. Just wanted to add..

The hardest part about this question is figuring out what the question is really asking! A lot of students rephrase this question as "is x divisible by n?" That's not necessarily wrong, but it doesn't actually help to answer the question, because n is just some factor, and not a specified one.

Whenever you're struggling to rephrase a DS question, here are a few things you can do:

- look for CLUE WORDS. Here, the word "factor" is a clue that we're talking about divisibility. If we want to know if it has a factor - if it's divisible by something - that will depend on prime factors. Greater than 1 is also a clue that we're talking about PRIMES.

- think about the "NO" CASE. When would an integer NOT have a factor between 1 and itself? When it's prime! The question must be asking about primes.
Ceilidh Erickson
EdM in Mind, Brain, and Education
Harvard Graduate School of Education
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