Leave an eternal 1089, just say a three-digit number, then subtract the inverted digit from this number and add the previous difference to get the income. The answer must be 1089, I don't think the landlord can understand it? Hehe, for example, reverse a random number 624 to 426 624-426 =198198 to 891=1089, which is a fixed value. Then you pick up a Modern Chinese Dictionary and tell your classmates that I can guess what the first word in the first line of page 1089 is. Actually, I just need to look ahead. Note: If the three-digit number is123123-321is negative, then I think I have made myself clear!