Consider the set of 6-digit integers, where leading 0's are permitted. Two integers are considered to be "equivalent" if one can be obtained from the other by a rearrangement (permutation) of the digits. Thus 129450 and 051294 are "equivalent". Among all the 10^6 six-digit integers:
a. how many non-equivalent integers are there?
b. if digits 0 and 9 can appear at most once, how many non-equivalent integers are there?
c. Generalize your results to (a) and (b) for n-digit integers.
My brain is going to explode~~ Thank you for the helps.