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Four travelers come to a narrow bridge at night. They have but one torch among them....

Four travelers come to a narrow bridge at night. They have but one torch among them. Knowing that they can only cross one or two at a time and none can cross without a torch, what is the shortest time they can cross if traveler A takes 1 minute to cross, traveler B takes 2 minutes to cross, traveler C takes 5 minutes to cross and traveler D takes 10 minutes to cross.

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Answer #1

Since 'A' has the minimum travel time, he need to accompany each one of B, C and D to cross the bridge, so that B, C and D will get to cross only once and A gets to cross a total of 5 times (1 for accompanying each of B, C and D and 2 more for crossing backwards to pick other one, after he accompanied one).

A BCD 2. Cp AD

The shortest time is thus 19 minutes.

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