Problem Solving

perwinsharma wrote:To find the number of primes till 'x', use the formula x/(ln x - 1)

where ln = natural log of 'x'

Just to be clear (for those just beginning their prep), you need not have any knowledge of logarithms to ace the GMAT.
alexandrabiorka, this question is clearly out of scope for the GMAT, so whatever resource you are using, it may be steering you in the wrong direction.

Cheers,
Brent

ASIDE: perwinsharma, your formula provides only an APPROXIMATION of the number of primes.

Hi alexandrabiorka,

I agree that this question isn't written in proper GMAT-style, although some of the concepts behind this question will show up on Test Day. This problem is more about figuring out the NON-PRIMES than the primes. The answer choices are "spread out" enough that we can do a little bit of work and eliminate all of the wrong answers.

We have the numbers 1 to 10,000 (I assume it's meant to be inclusive). What obvious numbers do we know that are NOT prime?

Evens (other than the number 2)
ANY multiple of 3 (other than the number 3)
Any multiple of 5 (other than the number 5)

There are 5,000 evens
There are 3333 multiples of 3 (although half of them are even so they were already "counted", so we really have 1666)
There are 2000 multiples of 5 (although half of them are even; of those that are odd, about a third are multiples of 3, we have 666)

With just these 3 options, we have:
approximately 5000 + 1666 + 666 = OVER 7,000 NON-primes.

10,000 - (over 7000) = LESS than 3000

The only answer that makes sense is A

GMAT assassins aren't born, they're made,
Rich

Nice work, Rich.
The Process of Elimination technique eluded me!

Cheers,
Brent