multiplicity that they appear with in any one number, and then multiplied
them together to find the required number.
It is clear that as 4=2*2, then 2 must appear at least twice in the
factorisation of the desired number, and as it appears no more times
in any one of the factorisations of the numbers in the list it must appear
exactly twice in the prime decomposition of the desired number.
Same for 9=3*3.
The only prime factor of a number in the list now unaccounted for is 5,
so we must have one of those as well.