How long is the forest, at what point did he enter, and at what angle?
If it were presented as a riddle, then jumping in to the arithmetic is the last thing you want to do. Even when it’s not, it’s always worth stopping and thinking. The whole point of riddles like this is to get people to pay close attention to the wording and not make assumptions about what they’re actually being asked to do. This can just be poor communication as per the XKCD strip, but that strip would have a lot more force if Monroe had managed to quote the riddle accurately.
He has his riddler saying “There are three words in the English language that end in -gry”. That’s not the riddle. The riddle is “Angry and hungry end in -gry. There are three words in the English language. What’s the third word?” Presented like that, it’s clearer that the whole “-gry” thing is a red herring to distract you from a much simpler question. Similarly, in the OP’s riddle the detail of the box’s dimensions is a lure to get you thinking about arithmetic, rather than asking why the box was specified to be empty.
“The yolk of an egg is/are white”; “German plane/French passengers/Switzerland crash - where do they bury the survivors?”; “Everything in the bunglalow is gold - what colour are the bannisters?” - they all work on the basis of distracting you with irrelevant details so you miss the obvious question.
That isn’t to say the OP’s riddle is a good one - it’s not because there are so many alternative trick answers. But there are lots of types of puzzle and brainteaser and just because it didn’t have the words “Trick Question” flashing above it doesn’t mean you should accept it at face value as a straight arithmetic problem.
Pretty much. I had never heard it before, but since the OP presented it as a riddle, it seemed clear to me that we were looking for a “catch” or joke answer here. I actually came up with the answer “zero” instead of the “one” the riddle maker was angling for, but got the jist of where the “riddle” was.
As a riddle/trick question, I do agree it’s a bit of a piss poor one.