You are on a game show and the host of this game show displays three sealed envelopes.
 One envelope contains $1,000
 One envelope contains $500
 One envelope contains $100
The host does not know which envelope is which.
You pick one envelope. Its contents are not shown to you.
The host instead opens one of the other envelopes. It contains $500. Remaining are the $100 and $1,000 envelopes.
You are given a choice  do you stay with your original envelope choice or do you pick the other envelope? Why?
Answer, 9.23.24
This is a twist on the classic Monty Hall problem. A similar concept, but where the host does indeed know the contents of each envelope (or door). After the contestant's choice, the host reveals a lower value prize and the contestant is tempted to switch doors. Paradoxically, the contestant should always choose to stay with their first choice. The Wiki article on this has multiple logical explanations.
In the scenario above, the host does not know which envelope contains what. This is a conditional probability scenario  your chances at $1,000 are 50/50 and you might as well flip a coin to pick.

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