Originally Posted by ultramarina
I agree that it's cool! DD9 is really into math tricks and trying to invent them. BUT I would also wonder a bit, like puffin said, if it is a little odd that he is needing to do this for "to 20" facts like 15-7? Like maybe he is compensating for an issue with facts? For 23-7, it seems okay.

No. You should not be worrying. It's good.