The Three Chests

[Context: the late Raymond Smullyan's The Riddle of Scheherazade: And Other Amazing Puzzles takes place in a setting very like the Thousand and One Nights, except Shahrazad (or whatever Romanization you prefer) has failed to slake the mad king Shahryar's bloodlust and now she has to entertain him with puzzles to save her skin and the kingdom.]

Scheherazade began: "Auspicious King, Abdul the Jeweler has in his home three chests of drawers; each chest contains two drawers. In one of the chests, each drawer contains a ruby. In another of the chests, each drawer contains an emerald, and in the third chest, one drawer contains a ruby and the other drawer contains an emerald. Suppose you pick one of the three chests at random and open one of the drawers and find a ruby. What is the probability that the other drawer in the same chest will also contain a ruby?"

"Let me see now," said the king. "Oh yes, the chances are fifty percent." 

"Why?" asked Scheherazade.

"Because, once you open a drawer and find a ruby, then the chest with both emeralds is ruled out, and so you have either hit the mixed chest, or the chest with the two rubies, and so the chances are even."

Was the king right?

(Source: The Riddle of Scheherazade: And Other Amazing Puzzles by Raymond Smullyan) 

The king was not right but perhaps on the right track. It is true that the chest with two emeralds is ruled out entirely. What he missed is that there are not two but three possible events: one in which the mixed chest has a ruby picked out and two in which one or the other drawer of the chest with only rubies is opened. If that doesn't make immediate sense, consider whether the king's postulated 50% chance would apply if the chest of only rubies had ten drawers instead of only two!