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Quick quant question

captiv8edcaptiv8ed Registered User Posts: 63 Junior Member
edited September 2009 in GRE Prep
I ran into this one on a CAT practice test

All the prime numbers from 1 to 1000

How the heck do you figure that out?
Post edited by captiv8ed on

Replies to: Quick quant question

  • aussiek517aussiek517 Registered User Posts: 325 Member

    Start by dividing 1000 by 2. Now you know that there are at least 500 non-prime numbers. Then divide 100 by 3 and take half of that amount (because half are also multiples of 2). Keep going if you want, but you can be pretty confident after only doing 2 steps.

    Multiples of 2: 500 non-prime numbers
    Multiples of 3: somewhere around 130 non-prime numbers
    1 is also a non-prime number

    You already know that you can't have more than 370 prime numbers. You have a bunch of 5s in there (some are multiples of 3), multiples of 7s that you haven't accounted for, etc., so the number is almost definitely less than 333. If you wanted to feel really confident about your answer, you could also count the numbers that end in 5, making sure that you're not overlapping with your multiples of 3.
  • captiv8edcaptiv8ed Registered User Posts: 63 Junior Member
    Awesome!! Thank you so much
This discussion has been closed.