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Question

Four people need to cross a bridge at night. They have one flashlight for the four of them, but the bridge is only wide enough for two to cross at a time. Because it's so dark, anyone crossing the bridge must do so with the flashlight. Person A can walk across in 1 minute. Person B takes 2 minutes; person C takes 5 minutes, and finally person D needs a full 10 minutes to cross. Naturally, if two people are crossing the bridge, they move with the speed of the slower person. What is the shortest amount of time it will take for all four to get to the other side?
  1. 17

  2. 15

  3. 18

  4. 8


Solution

The correct option is A

17


The shortest time is 17 minutes. The key to this problem is that to minimize the time spent; we need to have persons C and D to cross together. However, to avoid one of them having to make the trip back, we need to ensure that when they cross, someone is waiting for them on the other side to take the flashlight back.

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