The Two Envelope Paradox
One envelope has twice as much money as the second one. Gerry does not know which envelope contains the larger amount.
He takes one of the envelopes, counts the money, and is offered the chance to switch the envelope.
He thinks "If the amount of money in the chosen envelope is X dollars, then the other envelope contains either 2X of 0.5X dollars, with equal probability of 0.5. The expected value of switching is 0.5 (2X) + 0.5 (0.5X) = 1.25X. This is greater than the value in the initially chosen envelope. It is better to switch."
What is your advice?