Autumn, Melanie, Aaliyah and Gracie 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.
Autumn takes 3 minutes, Melanie takes 5 minutes, Aaliyah takes 8 minutes, Gracie takes 11 minutes. What is the least amount of time for all four to walk through the tunnel? 
Notes: 
Example:
If Aaliyah and Gracie walk through the tunnel together it will take 11
minutes, If Aaliyah then walks the torch back through the tunnel a total of 19 minutes will have passed. Continue till all four are at the other end of the tunnel.
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29 minutes:
Autumn and Melanie walk through taking 5 minutes
Autumn walks back with the torch taking 3 minutes [8 minutes total]
Aaliyah and Gracie walk through taking 11 minutes [19 minutes total]
Melanie walks back with the torch taking 5 minutes [24 minutes total]
Autumn and Melanie walk through taking 5 minutes [29 minutes total]
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