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math "trick" - Karen's Musings
Random Rambling
math "trick"
received this in an email today:

1. Grab a calculator (you won't be able to do this one in your head)
(first of all, let me say I'm insulted by that. I tried and I did this in my head without any problem, though it certainly took longer)
2. Key in the first three digits of your phone number (NOT the area code)
(second, let me say this is also ridiculous. It works fine with the area code. It's just a less interesting answer)
3. Multiply by 80
4. Add 1
5. Multiply by 250
6. Add the last 4 digits of your phone number
7. Add the last 4 digits of your phone number again.
8. Subtract 250
9. Divide number by 2
Do you recognize the answer?

So yeah, it's cool. And I'll never figure out how people work these things out. I've tried to create similar "tricks" but have never been able to do it. It irritates me that other people are so much smarter than me, and it's unfair that it irritates me. I should get over myself. I'm not all that, and I know that, I just wish I were! :)

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14 comments or Leave a comment
mamadeb From: mamadeb Date: August 26th, 2005 12:45 pm (UTC) (Link)
It worked.

And then I wrote down the equation and figured out how it worked. Very cool.
estherchaya From: estherchaya Date: August 26th, 2005 12:56 pm (UTC) (Link)
Oh I can figure out how they work *after* the fact. It just annoys me that someone else can work out the equation to make it work in the first place. Like I said, I shouldn't be annoyed; it's not fair of me. It's a completely inappropriate response from me and totally irrational. But still, I'm irritated. :)
leahmiriam From: leahmiriam Date: August 26th, 2005 01:59 pm (UTC) (Link)
Don't be annoyed, someone has more extra time on their hands. Did you get new glasses? When I had to quickly get off the phone yesterday it was due to Joel slipping down a few stairs. He's ok, just a klutz like me!
estherchaya From: estherchaya Date: August 26th, 2005 02:01 pm (UTC) (Link)
leahmiriam From: leahmiriam Date: August 26th, 2005 02:05 pm (UTC) (Link)
I saw. Sorry about that!
mamadeb From: mamadeb Date: August 26th, 2005 02:04 pm (UTC) (Link)
Nod. But I'm happy I can reduce the equation. I know I don't have a puzzle brain - it's as foreign a language to me as music. I just accept that others can do it.
estherchaya From: estherchaya Date: August 26th, 2005 02:08 pm (UTC) (Link)
I have a puzzle brain for solving puzzles just not for creating them.

Same with music. I can dissect and analyze music, but I was a sucky composition student (I still have some of my lousy compositions from my former life as a music major and they are really embarrassing)
mamadeb From: mamadeb Date: August 26th, 2005 02:18 pm (UTC) (Link)
When I said foreign language, though, I meant it. I'm...not quite tone-deaf. I can hear it, sort of (I assume I'm missing something, though, and instrumentals leave me cold unless I spend the listening time trying to identify the instruments.) And I do not understand how people can reproduce it with instruments at all.
From: have_inner_lady Date: August 26th, 2005 01:00 pm (UTC) (Link)
Piff. Sure you could have come up with that if it mattered and if your life didn't involve 12 other simultaneous things. All it says is:

Type in your first 3 digits. Multiply by 10,000 to, you know, move them over.

Now add your last 4 digits to fill in those zeroes.


The rest is just them trying to pretend it's cooler than it is. Bleh.
estherchaya From: estherchaya Date: August 26th, 2005 01:29 pm (UTC) (Link)
Oh I know how it worked... I mean... I can work the algebra out.

For some reason, my brain doesn't allow me to do the reverse... the pretending it's cooler than that part.
xiphias From: xiphias Date: August 26th, 2005 01:17 pm (UTC) (Link)
Yeah, like a lot of people, my reaction to these is to write down the algebra, simplify the equation, and then plug in the values. That takes a lot of the mistery out of it.

I mean, I look at that, and see: ((250(80x + 1) + y + y) - 250) / 2

Which is, obviously, (20,000x + 250 + 2y - 250) / 2

Which is 10,000 x + y.

All you need to do is figure out what result you want, put it into algebra, and then stick in a bunch of weird stuff that all cancels out.

Actually, that kind of sounds like fun. Wanna make one up right now?
estherchaya From: estherchaya Date: August 26th, 2005 01:28 pm (UTC) (Link)
I know it's just the weirdest thing because I can boil the algebra down like you just did (which is virtually identical to how I have it written down on a pad in front of me). But there's something in my brain that can't turn it back around. For some reason I can't obscure the algebra quite as artfully. Mine still ends up too obvious.
From: kahunadoctor Date: August 26th, 2005 04:22 pm (UTC) (Link)
it didn't work for mine!
glenbarnett From: glenbarnett Date: August 27th, 2005 01:57 pm (UTC) (Link)

I did the general case in my head as I read it; I've seen a lot of these and this was one of the easiest ones I've come across.

The ones I saw as a kid often constructed your age from your birthdate or vice-versa (but hidden in some other stuff that got added in and taken back out).

I remember a smart friend (but not so used to puzzles) being totally befuddled by a somewhat more clever one (it actually appears to be mind-reading, since it seems it couldn't possibly know the last thing you have to make up). Until I explained it. It performs a set of operations on a number that you start with, gets you to turn the answer into a letter, to think of a country starting with that letter, an animal starting with the second letter of the country name and then think of the colour of the animal, whereupon it tells you the colour you're thinking of. I imagine you've seen that one.

But in that case the sequence of operations takes any number to one of two possible results, and then treats those two differently, so that they both end up at 4. So you always choose a country starting with D, for which almost everyone says Denmark (why not "Djibouti"?), then Elephant (what about Elk? Emu? Ermine? Eel? Euro?). So they almost all end up at "Grey".

In general the more usual ones just rely on doing and then undoing the same simple operations (add, subtract, multiply, divide), with nothing much else going on. I've always thought that it might be worth creating one that's actually subtle. There are some really neat tricks you can put in that make it seem absolutely impossible to know what the outcome might be. One day, maybe, I'll have some time on my hands and make one where people with good algebra might still say "How the hell can that possibly work?".

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