Joseph, William, David and Nicholas 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.
Joseph takes 3 minutes, William takes 4 minutes, David takes 6 minutes, Nicholas takes 7 minutes. What is the least amount of time for all four to walk through the tunnel? 
Notes: 
Example:
If David and Nicholas walk through the tunnel together it will take 7
minutes, If David then walks the torch back through the tunnel a total of 13 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
22 minutes:
Joseph and William walk through taking 4 minutes
Joseph walks back with the torch taking 3 minutes [7 minutes total]
David and Nicholas walk through taking 7 minutes [14 minutes total]
William walks back with the torch taking 4 minutes [18 minutes total]
Joseph and William walk through taking 4 minutes [22 minutes total]
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