Three coins are tossed. If one of them shows a tails, what is the probability that all three coins show tails?

The contrast between your interpretation and that in the other answers points out that there is an ambiguity in the wording of the question, in particular in the use of "one". You interpret it as "the first". The other answerers interpret it as "at least one". Someone might interpret it as "exactly one" in which case the answer would be 0.

To elaborate on Henry's comment: suppose the three coins are tossed but I have placed cups over them so you cannot see them.

• If you are allowed to pick one cup and lift it, and after doing that you see the coin under that cup was tails, we are in your interpretation of the problem.
• If I am allowed to look under the cups (but you aren't) and I tell you afterwards: 'at least one coin shows tails' then we are in the other posters answer to the problem.
• If you pick a cup but are not allowed to look under it, and I (knowing the outcome of all three coins) lift a different cup, showing that the coin under that one is tails (and offer you to change cups even if that is irrelevant for the current question, but hey, perhaps we are in a game-show, who knows?), then you can only compute the probability of three tails under some assumptions about what is going on in my head (e.g. would I always show you heads if I had the chance?).

The important thing is that all three scenario's are consistent with the given that 'one of them shows a tails'. So in other words: the answer depends on how you interpret this given and you should go complain to the person who gave you this riddle.

Here is the chart of every possible outcome where there is at least one tails:

$$\begin{array}{c|c|c}1 & 2 & 3 \\ \hline H & H & T \\ H & T & H \\ H & T & T \\ T & H & H \\ T & H & T \\ T & T & H \\ T & T & T\end{array}$$

There are 7 possibilities (each equally likely) where at least one coin is a tails. Only one of them is all tails. The probability is $$\dfrac{1}{7}$$

Let E $=$ all three coins show tail.

Let F $=$ one of the coins show tail.

$E=\{TTT\}$

$F=\{HHT, HTH, THH, HTT, THT, TTH, TTT\}$

$$P(E|F)=\dfrac{P(E\cap F)}{P(F)}$$ where $P(F)=\dfrac78$ and $P(E\cap F)=\dfrac18$

Therefore, the probability that all the three coins show tails is $$P(E|F)=\dfrac{P(E\cap F)}{P(F)}=\dfrac{\dfrac18}{\dfrac78}=\dfrac18\times\dfrac87=\dfrac17$$