Question 95224: This is impossible. Please tell me I am wrong.
There's a family of 4 people, and they're fleeing to safety across a rickety bridge that's swept away by a flood in 17 minutes. The family members have 1 flashlight, which they must use every time they cross the bridge. The Grandfather will take 10 mintues to cross the bridge, the mother 5 minutes, the brother 2 mintues, and the sister 1 minute. No more than 2 family members can be on the bridge at any one given time. Do they make it? If so, is there more tha one way for them to make it across.Gran
Answer by bucky(2189)   (Show Source):
You can put this solution on YOUR website! The Brother and the Sister grab the light and run across the bridge. The brother being the
slowest makes them take his 2 minutes to cross.
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Once they are on the other side, the sister takes the light and goes back across the bridge.
This takes her 1 minute to do.
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She hands the light to the Grandfather and the Mother who then cross the bridge. Since the
Grandfather is the slowest, it takes the Grandfather and the Mother 10 minutes to cross.
.
When they reach the other side, the brother takes the light and goes back across the bridge.
That takes him 2 minutes.
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Then the brother and the sister go back across the bridge together, and since the brother
is slower, it takes them 2 minutes to cross.
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With this plan of crossing the total time involved is 2 minutes + 1 minute + 10 minutes +
2 minutes + 2 minutes ... a total of 17 minutes.
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Another way very similar to the first way is to again have the brother and sister cross
together ... taking 2 minutes because the brother is slower than the sister.
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Next have the brother return across the bridge with the light. That crossing takes another
2 minutes.
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He gives the light to the Grandfather and the Mother and they cross in the 10 minutes it
takes for the Grandfather to cross.
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The sister then takes the light and in 1 minute she races back across the bridge to her brother.
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Finally the brother and the sister cross together in the 2 minutes it takes the brother
to cross.
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The total time involved in this series of crossings is:
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2 minutes + 2 minutes + 10 minutes + 1 minute + 2 minutes = 17 minutes
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Hope this helps you with the puzzle.
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