Before choosing any box, there are 13 out of 45 that have the decoder ring, that means your first selection has probability 13/45 of containing a decoder ring. 



After the first box has been chosen...

Assuming it had a decoder ring, there are now 12 boxes remaining with decoder rings hidden inside.  There are also now only 44 boxes left. Thus the probability of the 2nd box having a decoder ring is 12/44.   



The total probability is therefore  (13/45)*(12/44) = 156/1980 = 

 (about 0.0788 or  7.88%)



 

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