Think about it. If a number misses 0, it means that it is 10 times smaller than the original number. Remove the original number, that is, the difference between the final result and the correct result is 9 times that of the reduction.

Divide 80 1 by 9, and the reduced number can be calculated as 89. Then add a 0 to it, and the correct addend is 890.

It's good to ask if you don't understand, but you should use your head to ask again in the future. Don't be lazy ~ ~