DCBA is still a four-digit number, so A must be 1, otherwise ABCD×9 is not a four-digit number.
The unit number of D×9 is 1, so d must be 9.
B should not be carried forward, so the choice of B is 0 or 1. If 1 is selected, there is C*9+8= 1, because if B= 1, C*9 cannot be carried forward, and 9C+8= 1 is invalid, that is to say, B=
The unit number of c× 9+8 is 0, so c is 8.
Get ABCD= 1089.
1089×9=980 1
Please don't make copies downstairs.