(a) There are k kinds of postcards, each with a limited amount. Let there be ai copies of the i-th postcard. How many ways are there to send these postcards to n friends? (Each friend can receive zero cards, as well as multiple copies of the same card.)
(b) A particular random number generator is able to give a bunch of random integers from 0 to 65535. How many numbers do you need from this generator to guarantee that there exist two subsets of integers that sum to the same number? Show your steps.