Avery, Andrea, Leah and Madeline 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.
Avery takes 2 minutes, Andrea takes 4 minutes, Leah takes 7 minutes, Madeline takes 10 minutes. What is the least amount of time for all four to walk through the tunnel? 
Notes: 
Example:
If Leah and Madeline walk through the tunnel together it will take 10
minutes, If Leah 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
24 minutes:
Avery and Andrea walk through taking 4 minutes
Avery walks back with the torch taking 2 minutes [6 minutes total]
Leah and Madeline walk through taking 10 minutes [16 minutes total]
Andrea walks back with the torch taking 4 minutes [20 minutes total]
Avery and Andrea walk through taking 4 minutes [24 minutes total]
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