Name: 21-card trick - Numberphile
Uploaded: 2020-03-27T18:16:37.000Z
Duration: 10 min 12 s
Description: Thousands of YouTube videos with English-Chinese subtitles! Now you can learn to understand native speakers, expand your vocabulary, and improve your pronunciation...

Future simple

Just a bunch of cards and I need someone to pick a card.

So let's so randomly pick a card out of there.

You're memorizing it, and you can stick it back in the pile here, anywhere, anywhere you like.

And just to be extra careful that I didn't see anything.

And then now I'm going to deal them out into three piles and for show.

I need youto remember this card so that you can mentally pass along that information to me, and we can help that card rise to the top.

Okay, so you point to which column you see your card.

I hope you remembering your card, because otherwise this trick might not work.

All right, one more time just to be safe.

And now, how about two piles from these ones?

I think I was going to get a little nervous.

I imagine the people in the audience are probably a little more excited.

I think maybe I'm a little more relieved.

So the first thing we need to know is the name of this trick is actually the 21 card tricks.

So the one piece of information I kept from you is how many cards I have.

And so you can really start thinking about this in terms of modular arithmetic.

And when I divide that number by some fixed number, like three, I can ask for which remainder does that number have?

His no remainder seven has remainder one So seven divided by three.

I can get three times to a six plus one so I can get one.

And then I can also have to sew like eight.

So I can think about dividing up my numbers.

Any integer divided by three into these three groups.

Where the remainder zero remainder one remainder too.

Okay, so everybody knows, and it's okay, because eventually I already was gonna know that, but okay, two of clubs.

So we stick it back in and just for show.

Add to the enjoyment of the randomness of it all.

You certainly my my dog could have shuffled them.

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21-card trick - Numberphile

stick

trick

immediately

audience