Let's calculate the probability that, in a class of N students, no one has the same birthday date (we'll ignore leap years). The N students are placed in a line, with the first student free to have any birth date. The second student must avoid that date, being restricted to 364 days of the year. The third person is restricted to any of 363 dates... with the Nth student restricted to 366 - N dates. The probability, Q, of there being no common birthday is thus