18. (3 points) There are two three-digit numbers, their sum is 999. If the larger number is placed to the left of the smaller number, the resulting number is exactly 6 times the number formed by placing the smaller number to the left of the larger number. The difference between the two numbers is $\qquad$.


18. (3 points) There are two three-digit numbers, their sum is 999. If the larger number is placed to the left of the smaller number, the resulting number is exactly 6 times the number formed by placing the smaller number to the left of the larger number. The difference between the two numbers is 715.
【Solution】Let the larger number be $X$, and the smaller number be $999-X$,
$$
\begin{array}{c}
X+(999-X) \div 1000=6 \times[(999-X)+X \div 1000], \\
1000 X+999-X=6000 \times 999-6000 X+6 X, \\
999 X+999=6000 \times 999-5994 X, \\
999 X+5994 X=6000 \times 999-999, \\
6993 X=5999 \times 999, \\
X=857 ; \\
999-857=142 ;
\end{array}
$$

The difference between these two numbers: $857-142=715$;
Answer: The difference between these two numbers is 715.
Therefore, the answer is: 715.