Since I know he knows how to do this, it was obvious that he was forgetting something. Nope! He was taught a new method today at school! So now, instead of knowing how to do it, and usually getting them all right, he's screwed up, confused, and frustrated by the new method.
Of course, this comes from Everyday Mathematics. The new method has the kids adding in the middle of a subtraction problem:
This is the big problem with Everyday Mathematics. They think it's important for kids to experiment with different methods, so that they can find the one they like best. Instead of picking one, simple, algorithm and working towards mastery, they keep throwing in new methods which--I swear--have the main purpose of confusing and frustrating the kids and flat-out preventing mastery.
Considering that EM was put together by Bill Ayers' education department at Chicago, I wouldn't be surprised if this actually was the goal. After all, math is hard! And if some kids can't get it, it really isn't fair that some do, and those who do go on to have challenging careers, stable lives, and become wealthier than average. Since we don't want that evil future inequality, let's just nip this dangerous math thing in the bud right from the start!
The more I see of Everyday Math, the more evil it looks.