[Rescued from my old blog.]
My youngest daughter wanted to do Singapore math today. Miquon Red is her main math text this quarter, but we add a bit of Singapore Primary Math 1B whenever she’s in the mood. We turned to the lesson on subtracting with numbers in the 30-somethings. The first problem was pretty easy for her:
30 – 7 = 
I reminded her that she already knows 10 – 7. She agreed, “10 take away 7 is 3.” Then her eyes lit up. “So it’s 23! Because there are two tens left.”
Wow, I thought. She’s catching on quickly. We went to the next problem:
34 – 8 = 
“Now, this one is harder,” I said. “But you know what 10 minus 8 is, right? So we could take one of these tens and–”
She waved at me to be quiet. I was just getting started on my standard speech about how to turn a tough subtraction like 34 – 8 into the easy addition of 2 + 4 + two tens left. Her mind was still on the last problem, specifically on the two tens and the seven.
“If you have 27,” she said, “and you add 3 more, you get 30. And 4 more is 34.”
“Um, yes, but…” I interrupted. She shushed me again.
“And then you can take away the four. And then you can take away the three. And then you can take away one more…It’s 26!”
She continued through the next page that way. For each problem, she started with whatever number struck her fancy, usually containing at least one digit from the problem before, and she added enough to get up to the 30-something number in the book. Only then would she deign to subtract the number in question. I don’t think she ever saw the point of the mental math technique the book and I were trying to teach, but she did have a lot of fun playing around with the numbers. In the long run, that is probably more important.