Question 875430
There are 12 C 3 = 220 ways to pick 3 locks from the pool of 12.


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There are 4 defective locks.


There are 12-4 = 8 non-defective locks.


There are (4 C 2)*(8 C 1) = (6)*(8) = 48 ways to pick 3 locks where 2 are defective and 1 is non-defective.


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Probability: (# of ways to pick 3 locks with 2 defective)/(# of ways total) = 48/220 = <font color="red">12/55</font>