More Math Series

I asked Supriya what comes next in (10, 9, 8). That was easy: "7." Next I asked, "10, 8, 6, ?" It took a few seconds: "4, 2, 0." Then I asked, "12, 9, 6, ?" Another couple of seconds, "3, 0." She is getting really good at this.

Then I wanted to know if she can count by 5, so I asked her what comes after (5, 10, 15). This time she needed more time, but she had the right answers, first 20 and then 25, a few seconds later.

She is doing all this in her head, mind you...