IBM Research | Ponder This | February 2012 solutions
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February 2012

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Letting q=1-p, the probability that a heads will first appear on one of those six tosses is p(1+q^13+q^14+q^18+q^19+q^22). To make the tossing fair, this probability must equal 1/2. Solving the equation gives only one real solutions: p = 0.499905242928506776611191007553...

Using a continued fraction, we find the best rational approximations to be 94,668,920 /189,373,729.

The answer to the bonus question is that the six numbers, interpreted as positions in the alphabet, give the letters that make up the word WATSON.

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