This problem is a step up from Problem #028 - hidden key. Can you tackle this one?
You and your best friend are locked in jail for no reason at all, but you are given the opportunity to escape. You are taken to a room that has four opaque boxes. The key to your cell will be put inside one of the boxes, and then a (regular) coin is placed on top of each box. You may pick a single coin and reverse its face up, and then your friend will enter the room.
When your friend enters the room you are not allowed to talk, and your friend must open a box. If your friend opens the box with the key, you are set free. Otherwise, you are locked for eternity...
What is the strategy that you and your friend should agree upon, so that your friend can always find the key?
Give it some thought...
If you need any clarification whatsoever, feel free to ask in the comment section below.
You can read the solution [here][sol] to compare with your own solution. You can also use that link to post your own solution in the comments! Please do not post spoilers in the comments here.
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