Archie, Betsy, Celia and David 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.
What is the least amount of time for all four to walk through the tunnel?
If Celia and David walk through the tunnel together it will take 10 minutes, If Celia then walks the torch back through the tunnel a total of 18 minutes will have passed. Continue till all four are at the other end of the tunnel.
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If you like this puzzle you may want to try River Crossing.