Probably of Numbers
If there are 10,600 people born in the United States each
day, what is the probability that one of them would have the same last
four digits of my social security number?
Consider the number of ways that the last four digits of a social security
number can be formed: 10^4=10,000. Each gets 9999 out of 10000 chances to
have other than your social security number. If the last four digits are
effectively random, then the probability that everybody gets other than your
digits is (9999/10000)^10600. This subtracted from one is the probability
that at least one person gets your digits.
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Update: June 2012