13. There are five cards below, each with a number: 00123. Using them, many different five-digit numbers can be formed. Find the average of all these five-digit numbers.
$$
\begin{array}{l} 
\text { [Solution] Solution: }(1 \times 12) \times 10000+(2 \times 3+3 \times 3) \times 1111 \\
= 1 \text { Solution: }(1 \times 12) \times 10000+(2 \times 3+3 \times 3) \times 1111 \\
= 120000+16665, \\
= 136665 . \\
(2 \times 12) \times 10000+(1 \times 3+3 \times 3) \times 1111, \\
= 240000+13332, \\
= 253332, \\
(3 \times 12) \times 10000+(1 \times 3+2 \times 3) \times 1111, \\
= 360000+9999, \\
= 369999,
\end{array}
$$

The average is $(136665+253332+369999) \div(12 \times 3)$,
$$
\begin{array}{l}
=759996 \div 36, \\
=21111 .
\end{array}
$$

Answer: The average of the five-digit numbers formed is 21111.
$$