A prime number is a positive number that has only two factors: 1 and the number itself.
Prime factors are prime numbers that are the factors of another number. For example, 5 and 7 are the prime factors of 35.
Think about the following to determine if a number is prime or composite:
Review this table of prime numbers between 0 and 1000:
There are multiple ways to assess whether a number is prime or not. The most obvious is to look for factors of a number—if you find that a number has factors other than 1 and itself, it’s composite! This is a simple process for some numbers, but this can be more difficult for larger numbers.
For example, is the number 621 prime or composite? Using the list of tips above, we can see that if a number’s digits sum to a multiple of 3, then the number is divisible by 3. Since 6+2+1 sums to 9, we know that 621 is not prime.
There are more divisibility tests which will enable you to quickly assess whether a number is prime or composite. Review the following rules:
Divisibility Test Rules
These rules should expedite the process of finding prime numbers. Try it out with the following practice.
Answers to Practice Problems