If the sum of the digits of a given number equals nine or a multiple of nine, nine is a factor of that number.
No. To find if a number is a factor of 3, add the number's digits. If the resulting number is a multiple of 3, then the original number is a multiple of 3. In 4339, the digits of the number added together is 19, which is not a multiple of 3.
Add up the digits of the given number. If they total a multiple of three, the number has three as a factor.
If the last two digits of your target number are a multiple of 4, 4 is a factor of the whole number.
21 or 35
450 or 900
If that is the case, then three is a factor of the original number.
No factor of 9 has three digits. 450 is a three-digit multiple of 150 (and of 9) with a digit sum of 9.
I think you might mean "multiple." If the sum of a number's digits is a multiple of 9, the original number is as well. If you want to know how you know if a number is a factor of 9, memorize the numbers 1, 3 and 9.
A factor is what goes into a number, a multiple is what a number goes into. 3 is a factor of 9 27 is a multiple of 9.