Six friends Al, Ben, Cal, Dan, Elmer and Frank want to cross a bridge. It is a very dark night but there is only one torch between the six of them. The bridge is narrow so that only two people at a time can cross it. It is known that:

Al takes exactly 1 minute to cross the bridge.
Ben takes exactly 2 minutes to cross the bridge.
Cal takes exactly 4 minutes to cross the bridge.
Dan takes exactly 7 minutes to cross the bridge.
Elmer takes exactly 12 minutes to cross the bridge.
Frank takes exactly 19 minutes to cross the bridge.

When two people cross the bridge, they do so with the pace of the slower person.

Determine the minimum amount of time it will take all six friends to cross the bridge.

Each time someone (or rather a pair)  crosses, someone must come back. It might as well be the fastest one. So Al's job will be to ferry each of the others across. 

Posted by Charlie on 2016-10-10 12:17:24