Factoring Integers: Problem 1 

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.

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