You don’t always have to make all numbers add up to 9.
The number of digits the rounded-up value will have will depend on how much you round up to.
If you round up 3898 to 10,000 you would have rounded up by 6102 which has 4 digits.
If you round up 3898 to 4,000 you would have rounded up by 102 which has 3 digits.
If you round up 3898 to 3,900 you would have rounded up by 02 which has 2 digits.
So in this scenario, you shouldn’t do the addition of 3 + 4 + 2 = 9.
Let me know if this clarifies.
When you round up 3898 to 4000 you will have to add a 3 digit number to 3898 to get 4000. So you don’t have to add the 3 + 4 + 2 to get 9.
The amount you round up will have one digit lesser than the original numbers.
Both 3898 and 4000 are 4 digit numbers so the amount you have to round up has to be a 3 digit number which is 102.