The factors of 15 are three and five. The factors of 15 are the numbers that 15 can be divided by to give a whole number. Because every number is divisible by itself and one, the numbers one and 15 are not usually defined as being factors of 15.
The numbers three and five are prime factors of 15, as they are both prime numbers. A prime number is a number that is only divisible by itself and one. Although 15 has only prime factors, many other numbers have both prime and nonprime factors. For example, two and three are prime factors of 12, and four and six are the nonprime factors of 12.