The prime factors of 55 are 5 and 11. Prime factorization means finding the prime numbers that can be multiplied together to produce the original number. The other factors of 55 are 1 and 55, but since neither are prime, they are not considered prime factors.
A prime number can only be divided by 1 and by itself. While the number of primes is infinite, and prime numbers can be very long, there are a few identifying characteristics they all share. With the exception of 2, all prime numbers are odd numbers. With the exception of 5, no prime numbers end in the number 5. With the exception of 3, no prime number is composed of digits that add up to a number divisible by 3.