To whoever says 1/2:
the three cards:
WW
BB
WB
we can immediately eliminate the WW to get 2 differnet outcomes. yes, two outcomes but they are NOT equally likely:
BB
WB
the black came from one of them. Since there are 3 blacks, 2 on the BB, 1 on the WB, it is twice as likely that the black came from the BB as opposed to the WB
the odds are 2/3
thats the easiest way I can think of explaining it