'proper' methods start with the lowest prime that divides into the
value, and uses that until it won't go. Then we use the next prime that
divides and so on.
Example: Find the product of the prime factors of 280
2 into 280 goes 140 and so on
35 is odd so doesn't divide by 2
3 does not divide exactly into 35 so we use 5
So the answer is 2x2x2x5x7 [or 23x5x7]