BUT - how old is your son? A trick shouldn't really be needed for the examples you gave, by the time the child's maths is good enough to manage the trick the basic facts should be pretty anutomatic. AND does he know what he is doing and why? The details you gave of his explanations sound like he doesn't understand well enough to explain well. If he is just doing it because it is his latest neat trick and he is that kind of person fine, if he is just doing rote manipulations on basic maths not fine.
He's 6.7 years old and in first grade. He's in a class that expects kids to learn to answer 100, 11-18 equations in five minutes. So far, he's done three of those timed tests and is at 83% correct.
I know he's using some of these tricks to get the answer instead of using strictly memory, but I also know it only takes him around three to four seconds to do the equations in his head in this manner. So...is it preliminary memorization?
