6. Recall that if you have a collection consists of n objects of which:
n1 are of type 1 and are indistinguishable from
each other;
n2 are of type 2 and are
indistinguishable from each other;
:
:
nk are of type k and are indistinguishable from
each other;
and n1 + n2 + ... + nk = n. Then the number
of distinct permutations of the n objects is n! / (n1! n2! ... nk!).