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Also, neat trick: If you need to add numbers higher than 10 you can take your shoes off to be able to do so.
Pfft, amateur hour. I count to eleven without taking my shoes off all the time.
Just not at my kid’s school any more.
When lecturing I used to take my shoes off. I was asked about it once and said, which is true, it was because I was nervous and it helped my anxiety. But I should have used this as my reply.
This is applicable to 9, e.g. 27 -> 2+7=9 -> divisible by 9
One of the rare cases when a factorial doesn’t lead to an absurdly huge figure. If that had been something like a “divisible by 12! rule” it would have been a lot spicier.
And people wonder why an American politician wanted to change π to be 3. Gawd our country sucks.
π=3
Source: Mahajan, S. (2014). The art of insight in science and engineering: Mastering complexity. The MIT Press. p. 18.
That page is also a goldmine for of numbers you can use for back of the envelope maths or trolling professionals of various kinds. Are you working with chemists? N_A=6 * 10^23 mol^-1 What about physicists then? c=3 * 10^8 m/s.
