Question

A computer password uses 5 letters where no letter may be repeated. How many passwords are possible?

Answer 1

5x4x3x2x1=120

Answer 2

The answers are 21!/5!, 26!/5!, and 26!/21!

Answer 3

\[\binom{26}{5}5! = \frac{26!}{5!21!}5! = \frac{26!}{21!}\]