D all of the above. Probably comes from a narrow conceptualization of privacy that ends at “do you know what my privates look like”, maybe stretching to “can you listen to me orgasm”. It wasn’t specifically the intent of the piece, but I remember when Last Week Tonight did a bit about data collection exposed by Snowden and demonstrated that Joe Public doesn’t conceptualize the loss of privacy well until it is placed in the context of “private parts”/peeping tom behavior, at least in the US context
I didn’t use this method as a kid, but I do use it or something like it pretty often to solve the math formula that my phone requires to turn its alarm off because that can go up to 15 and I don’t have above 10s 100% well memorized. I can get 10, 5, 1, and 2 of anything pretty quickly, so 11 is 10+1, 12 is 10+2, 13 is 10+5-2, etc. I don’t think it would have met the speed requirements of my times tables tests back in elementary school, especially because I was probably slower on my 2s, 5s, and addition back then, but 2-3 iterations is generally few enough that I can close the alarm before it gets too loud/annoying, even in a half-asleep state