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Coin tossing (Posted on 2004-06-11) Difficulty: 3 of 5
I threw a coin n times, and never got three tails in a row. I calculated the odds of this event, and found out they were just about even; 50%-50%. How many times did I throw the coin?

A second question: what were the chances of having not gotten three heads in a row either?

See The Solution Submitted by Federico Kereki    
Rating: 3.6667 (6 votes)

Comments: ( Back to comment list | You must be logged in to post comments.)
Answer to the second question | Comment 22 of 26 |
Now you'll have to treat HH, HT and TH separately.
P(n+1)=P(n)*(1/2 + 1/4*1/2 + 1/4*1/2)=P(n)* 3/4.
Therefore,
P(n)=3/4 *(3/4)^(n-3)=(3/4)^(n-2).
P(n)=50% -> n=4.4
  Posted by Ruopeng Wang on 2004-06-16 11:11:29
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