Rule 9:
To factor an integer, simply break the integer down into a group
of numbers whose product equals the original number. Factors are separated by multiplication signs. Note that the number 1 is the factor of every number. All factors of a number can be divided evenly into that number.
Problem 1: Factor 15 completely.
Answer. We have
Normally the 1 is left out. It is understood that 1 is the factor of every number.