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This is an old problem and we should have probably started our group with it but here we go...

Monty Hall Problem
You are on a TV game show and given the choice of three doors:

Behind one door is a car which you want to win and behind the others, goats.

You pick a door, say Door 1, but the host (who knows what's behind the doors) instead opens another a door which has a goat.

He then asks you, "You have one more option: do you want to pick another door or stay with Door 1 that you originally picked?" Is it to your advantage to switch your choice?
It is in your advantage to pick another door. When the game started you had a 33.3% chance to pick correctly. If you pick door 1 then the host shows a door which contains a goat that means the other door has a 66.6% chance of containing the car. Therefore it is in you best interest to switch doors.