ken quirici wrote: > There are 1M people and 100 coins. There are 2**100 possible > sequences of 100 coins. The chance that 1 out of the 1M will flip > the same sequence as you is (with ** indicating exponentiation) > > 1M * (2**-100) (if I remember correctly you add the > probabilities of independent events > to get the probability that they will > all occcur).
You *multiply* the probabilities of independent events to get the probability that they will all occur. (What you said was wrong, but what you *did* was correct).
You add the probabilities of mutually exclusive events to get the probability that one of them will occur.
The events here are not quite mutually exclusive. Your answer is off by the probability that more than one sequence will match (around (1M * (2**-100))**2).
Ignoring that is safe for practical purposes, but then so is ignoring the probability of one match. Both are zero for almost all practical purposes.