Thread: Obvious stuff I never new....

1. Obvious stuff I never new....

You go through life and come across information or understanding that's so obvious and ridiculously simple, but you never new.

I have one that really would have come in useful; percentages are reversible. So, 8% of 25 is the same as 25% of 8. It always works, with any pair of numbers. Why did I never know that? I saw it on a TV program about improving mathematical mental dexterity. You flip the percentage around to get the easiest way to work out the result.

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2. You're not the only one, Mick! I actually had to write it out: 25x.08 = 25x(8/100) = (25/100)x8 = .25x8 because multiplication is associative (or some such thing)

Now I'll be dizzy the rest of the day, thinking of more examples

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3. Oooooooo, people are really going to hate me now. After the nth time I tell them 'so what you're really trying to say is ...'
I like this!

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4. I've known that "percentage trick" since Mr. G drilled commutative, associative, and distributive properties into our skulls in 5th grade.
I would say 8% of 25 is the same as 25% of 8... and since 25% is a quartah, 8/4 = 2.

But here's something I learned from Bourns today... never really thought about it:

https://www.bourns.com/pdfs/OnlinePotentiometerHandbook.pdf
(Pages are out of order; see bookmark 20.)
When measuring MR, the test current must be no greater than the maximum wiper current rating of the potentiometer. High current can cause errors and will damage the potentiometer. Caution: Never use a conventional volt-ohm-milliammeter, VOM, to measure resistance parameters of a potentiometer. For the minimum resistance condition the wiper is near one end terminal. Little or no resistance is in the test circuit. In this case, overheating and burn-out can occur even at a low voltage.

D'oh. Luckily, I only use an analog VOM (in conjunction with a screwdriver) to determine pickup polarities.

-rb

PS:
Not trying to be snarky... but, ironically relevant in context of the thread title....
It's "knew", not "new".

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5. With finance, and other things, there is the 72 rule.
Divide your interest rate into 72 to find out how long it takes to double your money.
https://betterexplained.com/articles/the-rule-of-72/
Example 72/6% = 12 years.
T

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6. Originally Posted by rjb
PS:
Not trying to be snarky... but, ironically relevant in context of the thread title....
It's "knew", not "new".
I saw the typo as soon as I posted it, but for some reason I'm unable to edit the title. Unless someone knows otherwise......

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7. This is a quick method of multiplying two numbers as taught to Japanese kids. All you have to do is draw lines and count the intersections.

https://youtu.be/0SZw8jpfAk0

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8. That will keep me awake tonight.

Back in the early 80s I worked with a guy (kid, really) who threw me a challenge - I write down two numbers and he would multiply them in his head and give me the answer quicker than I could using a calculator. He never lost. I'm talking about any number of digits and never knew how he did it. Maybe he had a 'trick'

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9. We learned a lot of that stuff, I even read a book called something like Mathemagic. Lots of quick ways to do whatever. Problem was I had to remember all these quick tricks, I never did. So I'd run into the situation a year or two later, and I'd think, "I know there is a quick trick for this but I forget what it is."

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10. Maybe there's a trick for remembering the tricks?

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11. I think they say if you can't remember the tricks you are an old dog, or something like that.

Don't they say, "if you can't teach an old dog new tricks, but we keep learning new tricks, that's proof we're not old dogs."

sorry

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12. I am 70 now, but when I was in college, I still couldn;t recall all the tricks, and they were certainly already around then. And I was a physics major, lots of math.

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13. Impressive, thanks for posting.

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14. ... casting out nines

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15. Originally Posted by R.G.
casting out nines
Woah, reminded me of those 9 & 11 tricks, for instance decimal equivalent of 1/9 = 0.11111..... and 1/11 = 0.0909090909.... Comes in handy sometimes.

Didja know, all single integer multiples of 9, their components add up to 9?

Also, finding 2x & 4x multiples of 7 in the decimal equivalent of 1/7 = 0.1428 . That one falls apart in the 6th decimal (57 instead of 56) but close enough for rock n roll.

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16. Assuming that things you may not know includes non-math related things I offer the following:
If one is asked to sort out hard boiled chicken eggs mixed in with raw eggs without cracking the shell you can apply some physics. Just try to spin the egg on its end like a top. The hard boiled eggs will remain vertical and spin for a bit. A raw egg will fall on its side after less than one turn. If you do this with 9 eggs you will get 9 correct answers. Also works for any number of eggs between 1 and and beyond.

Good demo to do with curious kids.

Disclaimer - I accept no responsibility for broken eggs. You have been warned.

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17. Yep, done that test many times, always works. Though in the southern hemisphere, you have to spin them the other direction.

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18. Originally Posted by Enzo
Yep, done that test many times, always works. Though in the southern hemisphere, you have to spin them the other direction.
Said Mr. Coriolis

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