mystical ball

[danhayes]

how does mysticalball.com guess the sign I'm thinking of?

[NickleArse]

The possible answers are always the same. I haven't seen it for a while but I believe they are all factors of 9. All the symbols change each time you re-try it. The symbols next to all the possible answers are all the same. Therefore it always knows which symbol you will be thinking of even though it may be different from the last one.

[5029]

The calculation always gives you a multiple of 9, and it you look at the grid, all multiples of 9 have the same symbol

[NickleArse]

That's what I said?

[NickleArse]

Echo!!

[5029]

No, you said "factor" of 9, which is wrong. Factors of 9 are: 9, 1 and 3. The calculation will always give you a mulitple of 9, which is different from a factor

[Squirrel]

There's also a simple mathematical way of spoiling this trick:

The number has two digits, x and y. The number can therefore be expressed as 10x + y where x is the tens and y is the units.

The wizard guy tells us to add the digits together and deduct this from our original number:

(10x + y) - (x + y) = 9x

So you always get an answer of the first digit of your secret number, times nine. If you wanted a puzzle where the multiples of eight were the answer, you would do the same as above but also subtract the first digit (x):

(10x + y) - (x + y) - x = 8x

Well done to Nickel and 5029 for figuring out the trick.

[Think Deep]

Algebra at it's most beautiful, Squirrel

[guyasleep]

too bad that the grammar is not at "it's" most beautiful! :P