A three-digit number is subtracted from the number written with its digits in reverse order, and the answer comes out positive. Which of these can that answer NOT be?

If the digits are a, $b$ and $c$ the two numbers are $100a+10b+c$ and $100c+10b+a$, so the difference is $99(a-c)$ — the middle digit cancels out entirely. Every possible answer is therefore a multiple of 99, and since $a-c$ runs from 1 to 9 the complete list is 99, 198, 297, 396, 495, 594, 693, 792 and 891. Now 450 is a multiple of 9, which is exactly why it looks plausible, but 450 ÷ 99 is not a whole number. The other three are on the list: $198=99\times 2,297=99\times 3$ and $594=99\times 6$. Remembering only 'the difference is a multiple of 9' is the half-truth that lets 450 through.