Ella, Anna, Taylor and Kayla 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.
Ella takes 2 minutes, Anna takes 3 minutes, Taylor takes 5 minutes, Kayla takes 7 minutes. What is the least amount of time for all four to walk through the tunnel? 
Notes: 
Example:
If Taylor and Kayla walk through the tunnel together it will take 7
minutes, If Taylor 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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Teacher, do your students have access to computers such as tablets, iPads or Laptops? This page was really designed for projection on a whiteboard but if you really want the students to have access to it here is a concise URL for a version of this page without the comments: Transum.org/go/?Start=February8 However it would be better to assign one of the student interactive activities below. 

Here is the URL which will take them to Tran Tunnels, a mathematical adventure.