[marktwain]

In their example, I knew that 293 was seven less than 300, so I subtracted 300 from 263 then added back seven.

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Yes, I did stuff like that as well.

Isaac Asimov wrote a whole book about those sort of tricks.

https://www.amazon.com/Quick-easy-math-Isaac-Asimov/dp/B0006BLVLA#customerReviews

[Hugin]

Interesting. I was thinking about buying a copy for my grandson, then I saw the 500 dollar price tag. I don’t need math to know that’s too big a number for me.

And obviously I misstated the number in my solution, and left out part of my work. ;~)