



 
Hi Carol, To save myself some typing I am going to let A be the event that you choose one good bottle and one bad bottle. Then
Why is there not a third term there? The probability that you choose a good bottle on your first choice is 9/12. If you chose a good bottle then there are 11 bottles left, 3 of which are bad so the probability you choose a bad bottle at this point is 3/11. Thus
Can you finish the problem now?  


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