I remembered the concept of base 9 from high school but couldn't remember quite how to do the steps. So, I used this site:
http://mathforum.org/library/drmath/view/55726.html
So, basically, this is how I did it:
3762 + 8271.
First, you add the ones column. 2 + 1 = 3. 3 is less than 9 so 3 is the number to put in the ones column.
Then, you add the tens column. 6+ 7=13. However, 13 is greater than 9. So, you have to subtract 9 from 13 and carry one to the hundreds column. 13 - 9 = 4. So, you put the 4 in the tens column.
Then, you add the hundreds column. 7 + 2 + 1 (the one you carried over from the tens column) = 10. 10 is greater than 9. So, you have to subtract 9 from 10 and carry one to the thousands column. 10 - 9 = 1. So, you put the 1 in the hundreds column.
Then, you add the thousands column. 3 + 8 + 1 (the one you carried over from the hundreds column) = 12. 12 is greater than 9, So, you have to subtract 9 from 12 and carry one to the ten thousands column. 12 - 9 = 3. So, you put the 3 in the thousands column.
The only thing in the ten thousands column is the 1 you carried over from the thousands column. 1 is less than 9. So, you put the 1 in the ten thousands column.
Put them altogether and you get 13143.
Tested made this fabulous set for me!!! Isn't it great?