How many three-digit numbers are multiples of 9?

Step 1. The ends. $9 \times 12 = 108$ is the first three-digit multiple, and $9 \times 111 = 999$ is the last.

Step 2. The AP. $108, 117, \dots, 999$ with $d = 9$.

Step 3. Solve. $(n - 1) \times 9 = 891$, so $n - 1 = 99$ and $n = 100$.