### Instruction: Compute $81_9 - 72_9$.  Express your answer in base 9.### Response: To subtract in base 9, I need to use the same rules as in base 10, but with the digits 0 to 8 instead of 0 to 9.
I can start by subtracting the units digits: 1 - 2.
But I can't do that without borrowing, since 1 is smaller than 2 in base 9.
So I need to borrow one from the tens digit of 81.
That means reducing the tens digit by one and increasing the units digit by 9.
So 81 becomes 70 plus 10, and 10 - 2 is 8.
Now I can subtract the tens digits: 7 - 7 is 0.
So the final answer is 08 in base 9.
But I don't need to write the leading zero, so I can simplify it to 8 in base 9.