perplexus.info :: Probability : 2 coins

2 coins (Posted on 2016-03-18)

Suppose you have 2 identical-looking coins: one that is fair and one that comes up heads 75% of the time.
You randomly chose one of them, flip it 3 times and it comes up as H-H-T.

What is the chance that the coin you picked is the fair one?

Source: Project Euler

No Solution Yet Submitted by Ady TZIDON

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Simple Bayesian calculation

The fair coin comes up HHT with probability (1/2)*(1/2)*(1/2) = 1/8 = 8/64

The other coin comes up HHT with probability (3/4)*(3/4)*1/4 = 9/64

In my head, then, the probability of the fair coin has just become 8/17. Final answer.

proper calculation is

(1/2)*(8/64) / ((1/2)*(8/64) + (1/2)*(9/64)),

but I just shortcutted it as 8/(8+9)

Posted by Steve Herman on 2016-03-18 11:05:31

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