You are told there are two envelopes. One contains twice as much money as the other one. You pick one but are allowed to change your mind after picking it. (You are equally likely to pick the one with less money as the one with more money.)
To figure out how much on average the other envelope should contain, one might average x/2 and 2x because one is equally likely to pick one as picking the other. Since this comes out to 5x/4, one might always change his or her mind. But wouldn't this end up with one never making up his or her mind?
I think that you cannot do the average between x/2 and 2x because x doesn't refer to the same value.
We can assume that x is the lower prize so you can have x or 2x in the second box. So in the second box there will be an average of 3x/2 but you don't know if you already have x or 2x.