In probability theory, the birthday problem asks for the probability that, in a set of n randomly chosen people, at least two will share the same birthday. The birthday paradox is the counterintuitive fact that only 23 people are needed for that probability to exceed 50%.
I work in a company of about 80 people, and there are at least three sets of people with matching birthdates.
I say at least because I don’t know everyone’s birth date.