Question 53140
To fully factor a number, all the factors must be prime numbers.  Primes can only be divided evenly by 1 and themselves.  So, some prime numbers are 2, 3, 5, 7, 11, 13, 17, 19, 23... They go on and on!  A trick is that if any number, no matter how many digits are in that number, ends in (or has a last digit of) a 0, 2, 4, 6, or 8, the number can't be prime and can be factored -- because those numbers can always be evenly divided by 2.  Another hint is that if any number, again it doesn't matter how many digits long the number is, ends in a 5, then that number isn't prime as it can be evenly divided by 5.

So, for 45, ending in a 5, let's first divide by 5:  {{{45/5=9}}}.  So far we have factors of 5 and 9; 5 is prime, but 9 is not, so we continue to factor 9, which factors into 3*3.  Because 3 is prime, we have found all our factors of 45... 5*3*3.