I’m convince the answer is 0.5 but I keep getting it wrong, can anyone give me some hints?

[real dragon]

Helps please

[trim prism]

1. Wait patiently for a helper to come along.
2. Once someone helps you, say thank you and close the thread with:

+close

3. Feel free to nominate the person for helper of the week in #helper-nominations
4. Do not ping the mods, unless someone is breaking the rules.
5. If you're happy with the help you got here, and the server overall, you can contribute financially as well:

[lusty tiger]

you just count the numbers : $\ \frac{#B-# (B \cap C \cap D)}{#(B \cup \overline{B})}$