9.12 How many different four-digit numbers can be formed from the digits $0,1,2,3$, if each digit appears only once in the representation of the number?
9.12 The considered four-digit number can be represented as some permutation of the digits $0,1,2, 3$, where the first digit is not zero. Since four digits can form $P_{4}=4$! permutations and of these, $P_{3}=3$! start with zero, the desired number is $4!-3!=3 \cdot 3!=1 \cdot 2 \cdot 3 \cdot 3=18$.

Answer: 18 numbers.