12. (3 points) From the digits $1,2,3,4,5$, any 4 are taken to form a four-digit number. Then the average of these four-digit numbers is $\qquad$
【Solution】Solution: $5 \times 4 \times 3 \times 2=120$ (numbers),
$$
1 \times 4 \times 3 \times 2=24 \text { (numbers), }
$$

That is, $1,2,3,4,5$ each appear 24 times in each digit position,
120 different four-digit numbers can be formed;
$$
(1+2+3+4+5) \div 5=3 ;
$$

Then the digits in each position of the average number are 3, so this average number is 3333. Answer: The average of these four-digit numbers is 3333.

The answer is: 3333.