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Problem:

Alex flip a fair coin three times. what is the probability that she gets two heads given that the first is a head?


My solution is based on the argument that from the given information the sample space can be limited to four elements {$HHH,HHT,HTH,HTT$} Of these {$HHH,HHT,HTH$} satisfies the required event.Thus the probability is equal to $\frac{3}{4} = 0.75$.

But the given answer is $0.5$,where exactly is the problem in my approach ?

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    Maybe I just don't see the problem, but flipping a coin once, twice or multiple times won't change the fact that the probabilities for heads and tails are $0.5$. Also, the coin doesn't know that the first flip was heads. So you simply calculate the probability that you get one head with two flips which is $0.5$.2010-12-12

2 Answers 2

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Given that the first flip is a head, the problem is equivalent to asking, what is the probability of getting exactly $1$ head in $2$ flips. There are two possibilities, that the first flip is a head, and the second is tails, and vice versa. This gives the probability the given answer says.


To address your question more directly, you should use conditional probability. Let $A$ be the event that Alex flips exactly $2$ heads, and $B$ be the event that the first flip is heads.

Based on the sample space you wrote out, note $P(A\cap B)=2/8$, as the two possibilities are $\{HHT,HTH\}$. Also, $P(B)=1/2$, as you listed the four possibilities.

Now use the formula

$P(A|B)=\frac{P(A\cap B)}{P(B)}.$

I think your main error is that $\{HHH\}$ does not satisfy the condition that the first flip is a head, and that there are exactly two heads, and thus should not be included when calculating the probability $P(A\cap B)$.

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    +1,Thanks yunone that seems a valid explanation, and for the support of my previous argument here is another problem you could try `A couple has two children .Find the probability that both are boys,if it is known that older child is a boy.` Try doing this in either ways :)2010-12-12
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The problem here is that if the answer is half, then what the question means is exactly two heads, so three heads are not allowed. In your solution you included the case with three heads.

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    That's very true!2010-12-12