,----[ Ramanraj K ramanraj.k@gmail.com ] | Arvind, an old friend of mine, recently told me this story: | | One night, four travellers named One, Two, Five and Ten walking | through a forest, come to a deep gorge and find that they have to | continue their journey across a weak wooden bridge, that won't stand | the weight of more than any two of them at a time. The bridge has a | few missing logs and they had only one torch with them to guide them | across. Also, One takes 1 minute, Two takes 2 minutes, Five takes 5 | minutes and Ten takes 10 minutes to cross the bridge, due to their | individual abilities. What is the minimum time within which the | four friends could cross the bridge? | | I reasoned that One being the fastest, could take Two, Five and Ten | across in about 19 minutes as follows: | | Task ::: Time taken in minutes | ---------------------------------------------------------- | 1 and 10 take torch and walk across: 10 | 1 returns with torch : 1 | 1 and 5 walk across with the torch : 5 | 1 returns with torch : 1 | 1 and 2 cross with the torch : 2 | ---------------------------------------------------------- | Total time: ::: 19 minutes | | Arvind said that they could save two minutes with another plan. I | was sure there is a "catch" somewhere, because it appeared | impossible. Did 1 and 2 cross earlier, and did 2 throw the torch | from the other side? No, there are no catches, it is a purely | logical problem to be solved just the way I proposed, but his plan | would take two minutes less of time. How could they cross the bridge | in just 17 minutes? `----
Task ::: Time taken in minutes ---------------------------------------------------------- 1 and 2 take torch and walk across : 2 1 returns with torch : 1 10 and 5 walk across with the torch : 10 2 returns with torch : 2 1 and 2 cross with the torch : 2 ---------------------------------------------------------- Total time: ::: 17 minutes