Mackenzie, Faith, Sofia and Riley 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.
Mackenzie takes 3 minutes, Faith takes 4 minutes, Sofia takes 7 minutes, Riley takes 8 minutes. What is the least amount of time for all four to walk through the tunnel? 
Notes: 
Example:
If Sofia and Riley walk through the tunnel together it will take 8
minutes, If Sofia then walks the torch back through the tunnel a total of 15 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
23 minutes:
Mackenzie and Faith walk through taking 4 minutes
Mackenzie walks back with the torch taking 3 minutes [7 minutes total]
Sofia and Riley walk through taking 8 minutes [15 minutes total]
Faith walks back with the torch taking 4 minutes [19 minutes total]
Mackenzie and Faith walk through taking 4 minutes [23 minutes total]
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