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Here's what I already did-

List of possible outcomes:

1. HH
2. HT
3. TH
4. TT

Since we already know that one must be a tail, HH will not be included/eliminated.

Hence, possible outcomes:

 1. HT
 2. TH
 3. TT

Hence, the answer is 2/3.

Is my answer correct?

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    The probability we seek is one head and the other tail hence the event is {ht,th}. – 2017-01-25

1 Answers 1

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It depends what you mean. "Given that one is tails": do you mean:

  • "given that this specific one is tails" (in which case the answer is $\frac{1}{2}$), or
  • "given that at least one is tails" (in which case you're correct and it's $\frac{2}{3}$), or
  • "given that exactly one is tails" (in which case it's $1$, of course)?

You might like to look up the Boy or Girl paradox, by the way.

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    It should be "given at least one is tails". I just needed the confirmation. Thanks so much for confirming it! :) – 2017-01-25