Puzzle No. 4Posted on 22 January 2016
A king has been trapped alone in a castle. There is one switch in each of the 4 corner towers; and only when each of them is set to a correct (ON/OFF) position, will the main gate open. Every time the king runs from the central hall to any one corner tower, changes the switch, and returns to check the gate, it takes him 1 minute.
What is the least amount of time in minutes, in which the king can be guaranteed to escape from the castle?
(You may assume that all switches start in the OFF position)
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