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Flipping for Dollars (Posted on 2002-10-09) Difficulty: 3 of 5
We play a game as follows:

I place one dollar on the table. I repeatedly flip a coin. Each time the coin comes up heads, I double the money on the table. The first time the coin comes up tails, you take the money and the game is over.

What's a fair admission price for the game?

Would you play the game with me for $100?

  Submitted by Jim Lyon    
Rating: 3.9000 (10 votes)
Solution: (Hide)
The probability that the nth flip is tails is 1/2^n. The amount of money won if the nth flip is tails is 2^(n-1).

Mathematically speaking, the value of the game is:

1/2 * 1 + 1/4 * 2 + 1/8 * 4 + ...

which equals

1/2 + 1/2 + 1/2 + ...

So, mathematically speaking, the value of the game is infinite.

In practical terms, however, you'll soon be limited by the amount of money I have. (That is, there will soon come a point where I am incapable of doubling the money on the table.) So, the series ends at that point. In reality, the series only has as many terms as log2 of the money I have, and the value of the game is 1/2 that.

So, if I have about $1000, the value of the game is $5. If I have about $1,000,000, the value of the game is $10. If I have $1,000,000,000, the value of the game is $15. Even if I'm Bill Gates, the value is less than $20.

Comments: ( You must be logged in to post comments.)
  Subject Author Date
Some ThoughtsPuzzle ThoughtsK Sengupta2022-12-27 22:15:50
No Subjectcogitater2007-01-14 03:20:53
How much you playDan Porter2004-01-15 02:49:15
No SubjectSara2003-11-10 20:18:31
Some ThoughtsThe houseratsnstuff2003-10-14 05:30:05
Try thisRyan2003-05-07 08:21:13
No SubjectAlan2003-01-04 14:18:04
re SolutionCheradenine2002-10-14 03:02:08
re(2): something missingCheradenine2002-10-10 08:11:14
re: something missingCheradenine2002-10-10 07:37:38
something missingCheradenine2002-10-10 05:24:00
Some ThoughtsMy takefriedlinguini2002-10-10 03:45:11
re(3): Wait a second...(Another DOH!)TomM2002-10-09 20:08:27
Hints/TipsDoh!TomM2002-10-09 19:03:32
re(4): Wait a second...TomM2002-10-09 18:24:26
Some Thoughtsre: Is $100 high or low?TomM2002-10-09 18:12:19
Is $100 high or low?TomM2002-10-09 18:07:29
Some Thoughtsre(3): Wait a second...TomM2002-10-09 18:05:21
re(2): Wait a second...levik2002-10-09 16:44:53
re: Wait a second...Jim Lyon2002-10-09 11:49:16
Wait a second...levik2002-10-09 11:37:23
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