Compute $58_9 - 18_9.$ Express your answer in base $9.$
To do this, I can use the same algorithm as for base $10$ subtraction, but I need to be careful with the borrowings and the differences. Starting from the rightmost column, I have $8_9 - 8_9 = 0_9.$ No borrowing needed here. Moving to the next column, I have $5_9 - 1_9 = 4_9.$ Again, no borrowing needed. There are no more digits to subtract, so I am done. The final answer is $40_9.$ 