xkcd

by **Patashu** » Thu Apr 26, 2007 3:15 am UTC

You are sitting in front of a table, blindfolded and with heavy gloves on. On the table are 100 quarters, 20 of which are tails up. You can move around and flip the quarters but you cannot feel which side is which.

Your task? Separate the quarters into two groups so that the number of tails in both groups is equal. You are allowed to end with a different number of tails than you started with, as long as it's equal between the two groups you've made. Show a method that will work 100% of the time.

(You'll hit yourself once you get it.)

by **3.14159265...** » Thu Apr 26, 2007 3:16 am UTC

Did you check if hasn't been done before, in terms of some magic trick?

by **parallax** » Thu Apr 26, 2007 4:26 am UTC

Separate 20 coins from the other 80 and flip all the coins in the 20 group.
If there were X tails in the 20 group, then there are 20-X tails in the 80 group. After flipping all 20 coins, there will then be 20-X tails in the 20 group as well.

by **markkat** » Fri May 04, 2007 2:14 pm UTC

Separate them 50/50. -You didn't specify tails up or down.

by **markkat** » Fri May 04, 2007 2:18 pm UTC

Oops, is there supposed to be a solution thread?

by **Cosmologicon** » Fri May 04, 2007 4:43 pm UTC

I don't think it's necessary to conceal joke answers.

by **markkat** » Fri May 04, 2007 4:55 pm UTC

Cosmologicon wrote:I don't think it's necessary to conceal joke answers.

by **Spaz Funbag** » Sat May 19, 2007 1:32 pm UTC

So we're talking uniformly distributed, like

Code: Select all: HHHHHHTHHH HHTTHHHHHT HHHHHHHHTT THHHHTTHHH HHHHHHHHHH HHHHTTTTHH THTHHHHHHH HHTTTHHHHH THHHHHHHHT

?

And putting them on their sides does not count?

by **Maseiken** » Sat May 19, 2007 1:33 pm UTC

Throw the table through the window.

by **Spaz Funbag** » Sat May 19, 2007 1:43 pm UTC

I change them for a 20- and a 5$ bill, making those my two groups. Neither one has tails up, so i take the cash and spend it on ice cream.

by **pete** » Sat May 19, 2007 2:05 pm UTC

Remove the blindfold with your heavily gloved hands, or what parallax said.

Click link. Receive my face. · by **bbctol** » Sat May 19, 2007 5:16 pm UTC

Maseiken wrote:Throw the table through the window.

In a blindfold? What if it hits the wall?

by **Maseiken** » Sun May 20, 2007 2:28 am UTC

I was referring to the window that is created by throwing the table.[/i]

by **vaded_eyes** » Sun Jun 17, 2007 5:16 am UTC

parallax wrote:
Separate 20 coins from the other 80 and flip all the coins in the 20 group.
If there were X tails in the 20 group, then there are 20-X tails in the 80 group. After flipping all 20 coins, there will then be 20-X tails in the 20 group as well.

you dont know what the other eitghty is.. so how can u do this???

i think we shoulddd throw this out the newly made window...

by **semicolon** » Sun Jun 17, 2007 6:29 am UTC

You don't have to know. You just separate them into two groups, one 20 and one 80. The original problem said the groups didn't have to be the same size.

If the 20 coins you randomly selected included 5 tails, then that means there's 5 tails in the 20 group and 15 tails in the 80 group. If you flip every coin in the 20-coin group, this makes 15 tails. This always works, no matter how many coins with tails are in the randomly-selected group of twenty coins.

xkcd

Partitioning Quarters

Partitioning Quarters

Wording?

Solution thread.

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