Sadly, he's often one off. It's those tricky little errors that creep in... and of course, once he's figured it out, that it has to be that way, then it has to be that way.
I taught him the trick of adding up the answer from the ... damn, the correct word has escaped me. The number that you were supposed to take away plus the leftovers equals the original number. He thought that was a really, really cool trick. Trouble is, he hasn't got all of his original addition problems down either, after a certain level. 13 - 4 kept coming out as 8.
We finally had to take a fifteen-minute break from the frustration. When we came back, I got him started on repeating the plus-four tables: 4 + 0 =4, 4 + 1 = 5 ...
We had some frustration for a while, until I accidentally stumbled on telling him that what I wanted him to practice was the pattern, not figuring each one out by itself, but going with the pattern. So we went through the pattern, stopping to repeat individual problems over and over and over several times, until finally we made it to the real answer: 4 + 9 = 13!
For the next problems, he got the idea very fast. Finished quickly. Yay!
I gave him the idea that he could practice saying the patterns with his jumprope. He liked that plan.
He's all sore and feeling like his body's going to fall apart. I asked him what he'd done in PE today. They'd done stuff with the parachute. Yay parachute!
...after homework, we might wind up going and hitting the hot tub. Mmm, hot tub. Or not, depending on guests.