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(Divide out allowed primes)
To check if n is ugly, divide n by 2, 3, and 5 repeatedly (as long as they divide evenly). If n becomes 1, then n had no other prime factors. Return true.
If n does not reduce to 1, then n has a prime factor other than 2, 3, or 5. Return false.
Edge cases: n ≤ 0 is not ugly. Handle those first.
