If you take a group of people born in a non leap year you would need 366 people for a 100% chance that someone shares a birthday but only 23 people for a 50% chance that somebody shares a birthday?
In: Mathematics
For 2 people its 1/366 they share a birthday.
For 3 people. Its 2/366 (1 and 2 share a birthday or 1,3) + 1/365 (or 2 and 3 share a birthday)
For 4 its 3/366 (1,2 or 1,3 or 1,4) + 2/365 (2,3 or 2,4) + 1/364 (3,4)
Etc etc. Do this for 23 people and its around 1/2.
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