Kevin, Jack, Caleb and Justin 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.
Kevin takes 4 minutes, Jack takes 5 minutes, Caleb takes 7 minutes, Justin takes 10 minutes. What is the least amount of time for all four to walk through the tunnel? 
Notes: 
Example:
If Caleb and Justin walk through the tunnel together it will take 10
minutes, If Caleb then walks the torch back through the tunnel a total of 17 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
29 minutes:
Kevin and Jack walk through taking 5 minutes
Kevin walks back with the torch taking 4 minutes [9 minutes total]
Caleb and Justin walk through taking 10 minutes [19 minutes total]
Jack walks back with the torch taking 5 minutes [24 minutes total]
Kevin and Jack walk through taking 5 minutes [29 minutes total]
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access to computers? 

Here a concise URL for a version of this page without the comments.
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Here is the URL which will take them to Tran Tunnels, a mathematical adventure.