The new school year had barely started when I got a call from a friend who is an elementary school principal. She told me that every morning she announces the names of students who are celebrating a birthday. "One student noticed that we've already had two days on which no

In probability and statistics, special numbers are used to compute probabilities by counting the number of ways certain events can occur. The most famous are combinations and permutations. Both are used to count the ways to arrange or select items from a set. If a set contains n elements: A