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.
When you use an exclamation mark with a number, you’re actually implying it’s not a normal number any more. It’s a factorial!
Ok, so how does this work?
5!=1×2×3×4×5=120
6!=720
These numbers get really large. For example:
15!≈1.3×10^12
So, next time you see a headline with 2000! in it, you’ll know what to expect.
There are also double factorials (n!!) and iterated factorials (n!)!, and they aren’t the same thing. Just add more exclamation marks and you get multifactorial. Check wikipedia to see how spicy it gets.
