probability .1) or towards tails (in which case each toss of the coin will land tails with probability .9 and heads with probability .1). for each coin, you get to observe one toss, and then have to state whether you think it's biased towards heads or tails, and what is the probability that's the right answer.
let's say that you decide to follow this rule: after observing heads, always answer "the coin is biased towards heads with probability .9" and after observing tails, always answer "the coin is biased towards tails with probability .9". do this for a while, and it will turn out that 90% of the time you are 