Julia, Destiny, Morgan and Kaitlyn have to walk through a long dark tunnel. They only have one torch between them so a maximum of two people can go through the tunnel at any one time. Each person takes a different amount of time to get through the tunnel and if two people walk together they must walk at the rate of the slowest.
Julia takes 2 minutes, Destiny takes 5 minutes, Morgan takes 9 minutes, Kaitlyn takes 11 minutes. What is the least amount of time for all four to walk through the tunnel? 
Notes: 
Example:
If Morgan and Kaitlyn walk through the tunnel together it will take 11
minutes, If Morgan then walks the torch back through the tunnel a total of 20 minutes will have passed. Continue till all four are at the other end of the tunnel.
Topics: Starter  Logic  Problem Solving  Puzzles
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Previous Day  This starter is for 8 February  Next Day
28 minutes:
Julia and Destiny walk through taking 5 minutes
Julia walks back with the torch taking 2 minutes [7 minutes total]
Morgan and Kaitlyn walk through taking 11 minutes [18 minutes total]
Destiny walks back with the torch taking 5 minutes [23 minutes total]
Julia and Destiny walk through taking 5 minutes [28 minutes total]
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