I couldn't solve the problem:
"Consider two four-digit numbers, where the second is obtained from the first by writing it in the reverse order. Find the maximum number of digits 5 in the number, which is the absolute value of the difference of those two numbers."
Please help!
$7622$ and $2267$ give difference $5355$ , hence $3$ fives are possible. Since the difference must be disivible by $9$ , $4$ fives are impossible.
Hence the answer is $3$.