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> Yes, you can do that one. But that's just about the only fast trick there is. I dunno about that. For division, anyway, there's a bunch of fast tricks that give you a decent approximation (i.e. decent precision, maybe to the nearest integer) Someone recently was surprised that I worked out the VAT (Value Added Tax, 15%) on a very large number in a few seconds. It's because its 10% of the number plus `(10% of the number)/2`. It's easy to get 10% of any number. It's easy to halve any number. It's a fast trick because there's two easy operations. There's a bunch of similar "tricks": 1%, 10%, 25% and 50% are fast to calculate in your head (at most 2 easy operations, like `(half of half of N)`). Then you either add or subtract N. Or you multiply N by 2. At most three easy operations gives you 1%, 2%, 4%, 5%, 10%, 11%, 12%, 14%, 15%, 20%, 21%, 24%, etc To someone who doesn't know how you are getting the answer it might seem like you are a human calculator because you get so many of those quickly, and they don't see the ones you don't do in 3 easy operations (say, 13%, which is 10% + 1% + (1% * 2)). IOW, it looks like a very impressive trick, but it isn't. |
Did you not see the neighbor comment where I made it extra clear I'm talking about times tables in particular?
There are tons of math tricks! But the multiplication table of the numbers between 1 and 10 is mostly rote.