Monty Hall Paradox
In a game show, there are 3 doors. Two doors have nothing behind them, but one door has a brand new shiny red car. The game show host knows which door has the car. You pick a door, and before the host opens it, he opens a door that you did not pick, which has nothing behind it. Now you have the choice of switching to the closed door that hasn't been touched yet, or staying at the door you originally picked.
Which choice is better, if you want the brand new shiny red car?
This is not an original problem.
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1 solution
This is a repeat problem. It is the Monty Hall Problem.
But at the beginning, each door had a 1/3 chance of having the car. When you pick a door, it had 1/3 chance of having the car, so there is a 2/3 chance it is behind another door.
When one of the other doors is eliminated, the door that you didn't pick and wasn't eliminated still has another 2/3 chance of having the car, so if you switch, you will get it right 2/3 of the time.
So Switch!