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# A diversion

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Had computer problems a couple of days ago, so I decided to do the bigass "scan the drives for anything possibly wrong" thing. While that was going on, I decided to catch up on some scanning, as the big polyglot printer-scanner-fax-thingy is hooked up to Shelly's machine.

Today's entry is an interesting piece of history. It's a page from the company newsletter from C&C Bank School in Chicago. Banking was a bit more thought-intensive in 1925, as computers were still a few decades from being fixtures in banks, and the faster you could do operations on numbers the better. So shortcuts and tricks that helped you do stuff in your head was a must.

Here's a quickie that was discovered by my Grandpa Hattan (1907-1977) when he was just a 17 year-old kid.

Cool eh?

Quote:
 Here is a short rule in cross multiplying ... It applies to all problems with two figures equal to five, where the opposite two are alike.

I have absolutely no clue what they are tring to say.

It's a multiplication trick for figuring out the product of two 2 digit numbers (such as 38 x 28), where two of the digits add up to 5 (such as 3 and 2) and the other two digits are the same (for example, both 8s).

Back in the days before cheap electronics, people who worked with figures all the time liked to memorise rules such as these to quickly do arithmetic. I've read old maths books full of tricks like these. It makes me glad that I live in an era of pocket calculators [smile].

Yeah, now we have obscene optimization tricks involving modular arithmetic and the binary number form that people from the 20s wouldn't see the point of.

Ah, computers.

Ah ok that makes sense.

In the odd circumstance where I've had to do stuff like figure out 38 x 28 in my head I've done it by:

38 x 28 = (40 x 30) - (40 * 2) - (28 * 2).

i.e. approximate as 40x30. Then refine to 40*28. Then refine to 38*28.

That's pretty cool... When I multiply numbers, I press WindowsKey + R, "calc", And then type in the numbers. [wink]

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