Oh my. I used to do 17 + 35 = 35 + 17 = 35 + 20 + (20-17) = 55 - 3 = 52. And then I found out that my father used to think the same way, but not my mother, who

Oh my. I used to do 17 + 35 = 35 + 17 = 35 + 20 + (20-17) = 55 - 3 = 52. And then I found out that my father used to think the same way, but not my mother, who was a teacher a long time ago and just for few years. And my breakthrough for that didn't happen when using all those squares (for 1s and 100s) or rows (for 10s).