Question 1172380
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"at least one coin is tails" is another way of saying "one or more coins is tails"


Here are all the possible outcomes of flipping 2 coins
HH
HT
TH
TT
where H = heads and T = tails


The outcomes
HT
TH
TT
all represent cases when we get at least one tail. All we've effectively done is kick out HH


There are 2 cases when we get heads (HT and TH) out of three cases mentioned above.
The probability of showing heads, given this reduced sample space, is 2/3.


If we know there's at least one tail, then the probability of getting heads is 2/3.


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Answer: 2/3
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