[ Post a Response | Puzzles Forum ]
Posted by NickMcG on 14/9/2009, 7:49 pm, in reply to "Re: The Passageway"
Good one Denis.
I know from past experience you are probably the world's leading expert on these bridge/tunnel flashlight/card puzzles.
Ref: http://www.contestcen.com/tough.htm
Bridge at Midnight (contributed by Denis Borris)
Eleven people on a war games weekend come to a railroad trestle. It is midnight and there is no moon. Crossing is very dangerous because the ties are slippery and unevenly spaced, but they need to get over in the shortest possible time. The people are of widely differing ages and fitness. The time it will take the people to cross is 5, 10, 15, 20, 30, 30, 35, 45, 50, 55 and 65 minutes, respectively. Whenever 2 people cross together, they cross in the time of the slower person. Each person knows everyone's time.
They have just 2 small flashlights that each cast only enough light to allow two people to cross safely. Once they begin crossing, both flashlights are in constant motion, being carried across and back on the bridge. They are never held waiting for someone to arrive. The flashlights can be handed off at either end, but not part way. Crossers never turn around or stop on the bridge. The last sets of crossers all finish together. Also, the two people who need 30 minutes hate each other and will not cross together.
How long does it take for everyone to cross (A) if one person can carry both flashlights, (B) if one person cannot carry both flashlights? How is this accomplished?
Please submit all answers in the form of a 5-column table or schedule. The left column should be the time, starting at T=0, the second column the people waiting to cross, the third column the people crossing the bridge, the fourth column the people returning, and the fifth column the people who have already crossed.
Message Thread:
« Back to thread