Two Envelopes Problem
You are given 2 envelopes which are exactly the same and are told that one envelope has twice as much money as the first. You open the first envelope and find that there is $10 inside. Should you switch or should you take the $10, if you want to maximise your earnings?
In this case, we think that it does not make a difference if we switch or not, but actually, it does.
You took the envelope with more money.
The envelope with less money has 20.
Expected value = (Case 1 + Case 2)/2
Hence you should switch!
*Update: a reader spotted a fallacy in this, can you decipher what it is?