GMAT Prime Numbers

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GMAT prime numbers are the building blocks for so many questions.  It is a basic concept, but make sure you are very comfortable with prime numbers.  A prime number is a positive number that is only divisible by 1 and itself.  1 is not a prime number and 2 is the only even prime number (this is important to remember for GMAT questions).  Here is a list of some of the lower value prime numbers:

2  3  5  7  11  13  17  19  23  29  31  37

You should be able to recognize these very quickly, and after going through just a few problems these will become second nature.

Prime Number: A number that is only divisible by itself and 1

Composite Number: Numbers greater than 1 that are not prime.

The more difficult prime numbers to figure out are larger numbers.  For example, if you had to figure out if 79 is a prime number, what would the process be?  Follow the steps below:

    1. Approximate the square root of 79, which is between 8 and 9
    2. Determine numbers with which to test:  if 79 is composite (not prime) it will have a factor* less than or equal to 9 (the larger of the two integers around the square root of the number you are checking).
    3. Evaluate – 2,3,4,5,6,7,8 and 9 are not factors* of 79, so it is prime



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