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Ten white draws from coin-coloured balls

Question

A bag holds ten balls. Each ball was painted black or white according to a separate fair coin toss. You then draw a ball, note its colour and replace it, ten times in total, and every draw is white. What is the probability that all ten balls in the bag are white?

Short answer

About 0.0700.070. The number of white balls KK is Bin(10,12)\text{Bin}(10,\tfrac12) a priori and ten white draws have likelihood (K/10)10(K/10)^{10}. Bayes gives 1∑k=010(10k)(k/10)10≈0.0702\dfrac{1}{\sum_{k=0}^{10}\binom{10}{k}(k/10)^{10}}\approx0.0702.

Adapted from Wilmott, Frequently Asked Questions in Quantitative Finance, ch. 10 brainteaser 'Balls in a bag', p.351; question and answer rewritten by SelfMadeQuant.com.

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