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.

- A. Archie takes 3 minutes,
- B. Betsy takes 5 minutes,
- C. Celia takes 8 minutes,
- D. David takes 10 minutes.

What is the least amount of time for all four to walk through the tunnel?

Notes:

- All four begin at the same end of the tunnel.
- The torch must be walked back through the tunnel by at least one person, it cannot be thrown.
- The answer is less than 29 minutes!

Example:

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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