2! Came the answer almost immediately.
I meant to teach him how to solve these equations but had no time. I don't know what came over me, but I still went on to ask him such questions! To my surprise, he beat me to it. It seems he didn't need to be taught how. I threw him more similar questions, he solved it quickly without using any fingers. Of course, the questions I gave him were relatively easy. He was also able to solve double digit addition I.e. 9+?=18 or 6+?=12. I have no idea how he calculate it in his head.
But if you're wondering how I'd teach him these type of equations, I would rely on my fingers once again! For the above question, I would show him the fingers as I count from 15 to 17. I.e count 16 (stick out 1 finger), 17 (stick out another finger). Then I'll ask him how many fingers are there. Two fingers- and that's the answer!
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