### Instruction: Compute $81_9 - 72_9$.  Express your answer in base 9.### Response: I know that 81 in base 9 means 8 nines and 1 one, and 72 in base 9 means 7 nines and 2 ones.
So to subtract 72 from 81, I need to subtract 2 ones from 1 one and 7 nines from 8 nines.
But I can't subtract 2 ones from 1 one, so I need to borrow a nine from the nines place and add it to the ones place.
This gives me 10 ones in the ones place and 7 nines in the nines place.
Now I can subtract 2 ones from 10 ones and get 8 ones.
And I can subtract 7 nines from 7 nines and get 0 nines.
So the difference is 08 in base 9, which is the same as 8 in base 10.