b)
Perhaps I have the answer - using your algebra Chris :)
The factors of 20 are 1,2,4,5,10,20
Now if we are going to have the largest number of primes to make 20 factors we need the most integers greater than 1 that can multiply to give 20.
they are 2*2*5
So the answer is 3 unique primes (6 primes altogether). This is the maximum number of prime numbers that can divide this composite number with 20 factors.
a * b * c^4
c) So the smallest positive integer that has exactly 20 positive divisors will be
3*5*2^4
$${\mathtt{3}}{\mathtt{\,\times\,}}{\mathtt{5}}{\mathtt{\,\times\,}}{{\mathtt{2}}}^{{\mathtt{4}}} = {\mathtt{240}}$$
Thanks Chris - this was a real joint effort. Are you happy with the answer ? 