### Instruction: Compute $81_9 - 72_9$.  Express your answer in base 9.### Response: To subtract two numbers in base 9, I need to remember the place values and the rules for borrowing.
The rightmost digit is the ones place, and it represents how many units of 1 there are.
The next digit to the left is the nines place, and it represents how many units of 9 there are.
And so on.
To subtract in the ones place, I need to do 1 - 2.
But I can't do that, because 1 is smaller than 2.
So I need to borrow from the nines place.
Borrowing from the nines place means reducing the digit in the nines place by 1, and adding 9 to the digit in the ones place.
So 81 becomes 70 + 10, and 1 becomes 10.
Now I can subtract in the ones place: 10 - 2 = 8.
To subtract in the nines place, I need to do 7 - 7.
That's easy, it's just 0.
So the final answer in base 9 is 08.
But wait, there's a convention to drop the leading 0, so the answer is actually just 8.