h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

... or to put it the magician's way: "Pick any number between 1 and 10, multiply it times two and add 8. Then divide the whole shebang by two and substract the number you originally picked. Got that? Lemme see... What's your result? It's four, ain't it?"

So what is going on here, mathematically? Is it just plain luck that this happens to work for all the numbers between one and ten? (Integers, that is.) Why does it also work for twenty, but not for, say, 33?

Simplifying (2x + 8)/2 - x, we get:

(2x+8)/2 - x =
(x+4) - x =
x - x + 4
= 4

So it's not coincidence. You're taking something which is equal to 4 and showing that it is 4. It works for every number, even for 33.

(Grrr... I should not do even simple equations in my head after working seven 12 hour night shifts in a row ) Damnit, I forgot my maths so badly - does (2x+8)/2 in fact equal x+4? I.e., is (2x+8)/2 the same as 2x/2 + 8/2? I think I have to call upon 2legs for blaming purposes...

Strange coincidence that the answer is always equal to the number of yellow sweets.

You think that is bad! - I had teh guy at teh fruit and veg stall almost wetting himself with laughter as my brain tried to calculate the change from a simple purchase of two items, all costing whole 10 pence divisions of a quid - I got it so epically wrong seems my mathmatical abilities have not been improved any of late (and the posts above... m ake absolutely no sense whatsoever to my brain; Quite how I managed maths and statitiscs modules, at uni, is beyond me now!) 1+1= 3. err. two. its two isn't it?

My daughter bought something for €1.35 the other day, presented a €2 coin and the woman apologised that her cash register wasn't working, got out a calculator and worked out the change.

I'd have had to do the same! - yeh, I was always pretty rubbish with maths, but... now... seems as with anything/everything else its a bit of my brain that got a bashing from the drugs Well, if it doesn't return, guess I'll just have to stick a calculator app on my phone's homescreen, to avoid being ripped off in any shops I use

{Why does it also work for twenty, but not for, say, 33}

(2 x 33 + 8) / 2 - 33

= (66 + 8) / 2 - 33

= 74 / 2 - 33

= 37 - 33

= 4


So yes, it does work for X = 33

And more than that, you were thinking of a grey elephant.

Multiplying by two in one step, dividing by two in a later step, and finally subtracting the original number leaves you with zero. Adding the eight ensures that dividing it by two will give you four, and that the four will be all that's left. That's why four is always the result,.

*LOOKS ON IN AMAZMENT* ITS... JUST... *TRYS TO UNDERSTAND IT..... * MAGIC!

It's just basic math.

i KNOW! BUT... ITS SO BEOND ME RIGHT NOW ITS ... FUNNY = i ACTUALLY HAD TO THINK, EARLIER, ON HOW MANY SAUSAGES HALF OF SIX WAS, WHEN WORKING OUT HOW MANY i'D HAVE LEFT, AFTER LUNCH

{It's just basic math.}

I couldn't disagree more with that sentiment. Arithmetic is merely a window on to the beautiful world of propositional logic. This discipline underpins not only mathematics and science, but our philosophy too.

Icy North, how did you know about the grey elephant?

And for my next trunk...

"I couldn't disagree more with that sentiment"

I'[m sure what part of it you're disagreeing with, Icy. The part that locates it in math? If so, and if [as you seem to be doing] you see it as more of a problem in logic, that's fine with me. The idea that it's "basic?" Well then, what would you consider intermediate or advanced? Please know before hand, though, that I may not be clever enough to cope with anything more complicated than basic.

Sorry, I meant that I'm *not* sure what the disagreement consisted of.

You're right Paulh. I like arithmetic because it's basic. If you want something more complex, then how about statistics?

I actually like statistics. One of my favorite books was "How to lie with statistics," which is sadly out of print.

Strange thing is; I'm fine with stats; I can work them out, understand them, etc., etc., ; just so long as I'm using a calculator for any maths involved in doing teh actual maths involved in them; but, mental maths, I just can't do! - I got something like 82% in my uni module in mathmatics, and a bit higher in the one on statitiscs, at my BSc degree... (they were modules in teh biomedical degree) - just don't ask me to calculate change from a simple cash transaction at a market stall