A security PIN code is four digits long. Each digit could have the value 0 to 9. If an office building needs 10 security PINs, express the number of codes in logarithmic form with base 10.
If digits can be repeated as in $\displaystyle 9009$ then there $\displaystyle 10^4$ different codes.
If digits can not be repeated as in $\displaystyle 1079$ then there $\displaystyle 10\cdot 9\cdot 8\cdot 7$ different codes.
In either case choose ten of that number.