How do I know when prime factors are redundant?
Example: To find out whether x is divisible by 120, it's given that x is divisible by 12 and by 30.
We need three 2's, one 3 and one 5 for x to be divisible by 120, since x is divisible by 12, it's divisible by two 2's and a 3 and since x is divisible by 30, it's given that it has a 2, 5 and 3 in it's prime factorization. However, one of the 2's could be redundant.
Now I've found another example, which doesn't seem to use this concept:
Questions: "Is the integer x divisible by 36?"
1) x is divisible by 12
2) x is divisible by 9
So x needs two 2's and three 3's. Statement 1 gives it two 2's and a three and statement 2 gives it three 3's. Which is according to the answer sufficient for x to be divisible by 36. Why isn't one of the 3's redundant in this problem?
Is there any general rule when prime factors are redundant?