For me, what is interesting in that problem (besides the obvious flaw that people here discussed) is that the answer changes is the value A is observed. I.e. :

For me, what is interesting in that problem (besides the obvious flaw that people here discussed) is that the answer changes is the value A is observed. I.e. : If I have two envelopes, one of which has twice the money of the other, you pick an envelope AND YOU SEE the money inside, then it becomes better to switch (in expected money).