Photo by I. Peterson
June 30, 2004
Crossing a Rickety Bridge
Four people need to cross a river at night. A rickety bridge over the river can hold only up to two people at a time. There's one flashlight that must be used when crossing. After it's used, it must be brought all the way back to the people remaining on the near side.
Each of the four people takes a different amount of time to get across. If two people cross together, they travel at the slower person's rate. Annie takes 10 minutes to get across, Barry takes 5 minutes, Charlie takes 2 minutes, and Speedy takes 1 minute.
What's the shortest time that it would take all four people to end up on the far side of the river?
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First, Speedy and Charlie cross together.
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It can be done in 17 minutes.
First, Speedy and Charlie cross together (2 minutes).
Speedy returns with the flashlight (1 minute).
Annie and Barry cross together (10 minutes).
Charlie returns with the flashlight (2 minutes).
Finally, Speedy and Charlie cross together (2 minutes).
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