# A fair coin is tossed until both heads and tails apear [closed]

A fair coin is tossed until both heads and tails have been obtained. Let x = the number of tosses needed (for example, if the sequence of tosses is HHT, then x = 3; If the sequence is TTTTTH, then x = 6).

Find the expected value of x aka E(x).

I think E(x) = 1/P, where P is the probability of getting a heads or tails.

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What is your attempt? –  tylerc0816 Oct 2 '13 at 14:55
Look at the definition of the expectation. $E(x) = (0 \times \text{probability of stopping after 0 tosses}) + (1 \times \text{probability of stopping after 1 tosses}) + \ldots$. –  utdiscant Oct 2 '13 at 14:58

## closed as off-topic by Nate Eldredge, Thomas, Davide Giraudo, azimut, Vedran ŠegoOct 2 '13 at 15:44

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The waiting time after the first toss has geometric distribution, with mean $\frac{1}{1/2}=2$.
So the expected waiting time until each of head and tail has shown up is $3$.