Ma Xiaolan knocked on the door and came in.

"What's wrong, Sister Lan?" Jiang Xuan put down her pen and came out.

Ma Xiaolan held Xiuxiu's hand, and Xiuxiu kept her head down, not daring to look Jiang Xuan in the eye.

"Xiuxiu didn't go to her tutoring class today. She has a few math problems she can't solve, and I can't figure them out either. Her dad isn't home. Can you help her with them?"

"Okay, I'll take a look."

Jiang Xuan took the workbook and pulled Xiuxiu to sit on the sofa. "Come on, Xiuxiu, let's read the questions carefully together first."

How many three-digit numbers have a sum of 10 for their hundreds digit and units digit, and are divisible by 9?

Jiang Xuan picked up a pen and wrote down "ten" and "hundred" on the draft paper.

"The question says that the sum of the hundreds digit and the units digit is 10, which means that if the hundreds digit is 1, the units digit is 9, right?"

Xiuxiu nodded.

"Similarly, if the hundreds digit is 2, then the units digit is 8. How many combinations are there in total?"

Xiuxiu whispered, "Nine kinds."

"Yes, Xiuxiu is really smart."

Jiang Xuan wrote down all the combinations.

"Now let's look at the next condition: the resulting three-digit number must be divisible by 9. To be divisible by 9, the number must be a multiple of 9. For example, 18 is divisible by 9, and 27 is also divisible by 9, right?"

"Um."

Jiang Xuan wrote down 18, 27, 36, 45, 54, 63, 72, 81.

"Xiuxiu, take a close look at these numbers and see if there's a pattern among them?"