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:
- Approximate the square root of 79, which is between 8 and 9
- 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).
- Evaluate – 2,3,4,5,6,7,8 and 9 are not factors* of 79, so it is prime