# 4/13 -- High School I Circle: Counting (Prof. Po-Shen Loh, Carnegie Mellon University)

How hard can it be to count? The answer to that question may depend on how
much stamina you have. It might, for example, take an awfully long time to
count how many 125-element subsets there are of {1, 2, ..., 250} by simply
listing all of them. There are faster ways to calculate this number, but
even then, at first glance it may appear to require a substantial amount of
computation in order to determine even the last digit of "250 choose 125".
In this talk, we will find this last digit, and explore some related
topics. (Hint: the last digit is not zero - that would be too easy!)