You are rolling two fair dice, and you are blindfolded, after a certain roll, your partner tells you that you have rolled at least 9.
- What is the probability that you have rolled at least 11?
- What is the probability that you have actually rolled an 11?
If you have rolled in at least $9$, then the possible outcomes are $(4,5),(5,4),(6,3),(3,6),(4,6),(6,4),(5,5),(5,6),(6,5),(6,6)$ i.e. there are $10$ possible outcomes.
If you have rolled in at least $11$, then the possible outcomes are $(5,6),(6,5),(6,6)$ i.e. there are $3$ possible outcomes.
If you have rolled in exactly $11$, then the possible outcomes are $(5,6),(6,5)$ i.e. there are $2$ possible outcomes.
So the probability in case (1) is $\frac{3}{10}$. And the probability in case (2) is $\frac{2}{10}=\frac{1}{5}$.