A typical PIN (personal identification number) is a sequence of any four symbols chosen from the 26 letters in the alphabet and the ten digits. If all PINs are equally likely, what is the probability that a randomly chosen PIN contains a repeated symbol?

Explanation

Solution. A PIN is a sequence of four symbols selected from 36 (26 letters + 10 digits) symbols.

There are `36 xx 36 xx 36 xx 36 = 364 = 1,679,616` PINs in all.