18. (10 points) A three-digit number $A$ is such that the difference between the largest and smallest three-digit numbers formed by its digits is still $A$. What is this number $A$?

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【Solution】Solution: Let the three digits that form the three-digit number $A$ be $a, b, c$, and $a>b>c$, then the largest three-digit number is $a \times 100 + b \times 10 + c$, and the smallest three-digit number is $c \times 100 + b \times 10 + a$,

So the difference is $(a \times 100 + b \times 10 + c) - (c \times 100 + b \times 10 + a) = 99 \times (a - c)$.
Therefore, the original three-digit number is a multiple of 99, the possible values are $198, 297, 396, 495, 594, 693, 792$, 891,

Among them, only 495 meets the requirement, $954 - 459 = 495$.
Answer: The three-digit number $A$ is 495.