Kmm smart,007 tricky,they are important for us.

This question can be solved by Prob.tree:P(b&b/b)=1/2.
use condition Prob.also OK,but be careful!
S={gg,gb,bg,bb},E={bb},F={gb,bb} or F={bg,bb},not {gb,bb,bg}
We get P(E/F)=P(E&F/F)=(1/4)/(2/4)=1/2.
Thanks to Kmm and 007.Let me think Qn slowly.