Katherine, Alexandra, Rachel and Lily 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.
Katherine takes 2 minutes, Alexandra takes 3 minutes, Rachel takes 5 minutes, Lily takes 7 minutes. What is the least amount of time for all four to walk through the tunnel? 
Notes: 
Example:
If Rachel and Lily walk through the tunnel together it will take 7
minutes, If Rachel then walks the torch back through the tunnel a total of 12 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
18 minutes:
Katherine and Alexandra walk through taking 3 minutes
Katherine walks back with the torch taking 2 minutes [5 minutes total]
Rachel and Lily walk through taking 7 minutes [12 minutes total]
Alexandra walks back with the torch taking 3 minutes [15 minutes total]
Katherine and Alexandra walk through taking 3 minutes [18 minutes total]
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