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Appendix
Factoring with Prime
Numbers
Factoring is a skill that is developed with practice. The only surefire way to fac-
tor numbers into their prime factors is by trial and error. There are some num-
ber facts that will make our job easier. Some of these facts should be familiar.
- If a number is even, the number is divisible by 2.
- If a number ends in 0 or 5, the number is divisible by 5.
- If a number ends in 0, the number is divisible by 10.
- If the sum of the digits of a number is divisible by 3, then the number is
divisible by 3.
- If a number ends in 5 or 0 and the sum of its digits is divisible by 3, then
the number is divisible by 15.
- If a number is even and the sum of its digits is divisible by 3, then the
number is divisible by 6.
- If the sum of the digits of a number is divisible by 9, then the number is
divisible by 9.
- If the sum of the digits of a number is divisible by 9 and the number is
even, then the number is divisible by 18.