Explain the maths of this trick

[sstein]

Watched a Ted Talk video tonight of a guy who calls himself a mathimagecian, impressive stuff.

He did a series of tricks involving squaring numbers the audience shouted out quickly. He then took the largest number that resulted from the squares. It was 8649.

He then asked the 4 people to pick a random 3 digit number and multiply 8649 with their chosen number. He didn't know what the 3 digit number was.

This gave them either a 6 or 7 digit number. (It turned out they all had a 7 digit number). They then read out 6 of the 7 digits, in any order, and the guy told them what number they left out..

I realise this is a trick, but can some smart mathematician explain how it's done ?

Here's the video.

http://www.youtube.com/watch?v=M4vqr3_ROIk

-

Stuart

[PhantomHumper]

4

[mybrainhurts]

Eleventy six...

[speedfreaky]

sstein said:

Watched a Ted Talk video tonight of a guy who calls himself a mathimagecian, impressive stuff.

He did a series of tricks involving squaring numbers the audience shouted out quickly. He then took the largest number that resulted from the squares. It was 8649.

He then asked the 4 people to pick a random 3 digit number and multiply 8649 with their chosen number. He didn't know what the 3 digit number was.

This gave them either a 6 or 7 digit number. (It turned out they all had a 7 digit number). They then read out 6 of the 7 digits, in any order, and the guy told them what number they left out..

I realise this is a trick, but can some smart mathematician explain how it's done ?

Here's the video.

http://www.youtube.com/watch?v=M4vqr3_ROIk

-

Stuart

It's all to do with number theory and multiples of nine!with any number that is a multiple of nine you will find that the induvidual numbers also add up to nine.therefore ,if you have a number that you know is going to be a multiple of nine ,given all but one of the numbers you can guarantee to "Predict" the missing number!

easy really.....

[The_Cheeseman]

42 clearly

[ewenm]

As above, these tricks are nearly always based on generating a multiple of 9 and using the special features of those numbers to perform seemingly impossible feats.

[kambites]

ewenm said:

As above, these tricks are nearly always based on generating a multiple of 9 and using the special features of those numbers to perform seemingly impossible feats.

Odd phenomenon, although not hard to prove if you try. Any multiple of 9 has digits that add up to nine.

There are other similarly unlikely looking rules, but few simple enough to quickly calculate in your head.

[sstein]

Thanks,

Just tried it on a spreadsheet and it does indeed work!

Woo

-

Stuart