Question

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?

Answer 1

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

By the fundamental principle of counting, there are 36 × 36 × 36 × 36 = 36⁴ = 1,679,616 PINs in all. When repetition is not allowed the multiplication rule can be applied to conclude that there are

36 × 35 × 34 × 33 = 1,413,720 different PINs

The number of PINs that contain at least one repeated symbol

= 1,679,616 – 1,413,720 = 2,65,896

Thus, the probability that a randomly chosen PIN contains a repeated symbol is