Puzzles/Logic puzzles/Bridge Crossing/Solution

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Puzzles | Logic puzzles | Bridge Crossing | Solution

Let's name the four people:

  • A - the person who can cross the bridge in 1 minute
  • B - the person who can get across in 2 minutes
  • C - the person who needs 5 minutes to cross the bridge
  • D - the slowest and needs 10 minutes to get across

Now, to make it to the other side in 17 minutes, they do the following

  1. A & B cross first (total time: 2 minutes)
  2. A leaves B on the other side and returns with the flashlight (time: 1 minute; total time: 3 minutes)
  3. C & D cross with the flashlight(time: 10 minutes; total time: 13 minutes)
  4. B returns with the flash light (time: 2 minutes; total time: 15 minutes)
  5. Finally, A & B cross to the other side of the bridge (time: 2 minutes; total time: 17 minutes)

Here's a funny poem written by Karen Lingel:

FOUR HUNGRY MEN CROSS A BRIDGE by Karen Lingel, Physicist and Penguinist

Four men start out to cross the sea
And yet they all walk different speeds!
The first, a sprinter, he goes fast
He leaves the others in the past!
The second takes a bit more time
[Note to myself: think up a rhyme]
The third's a somewhat pokey man
He strolls along, sees what he can.
The last one is so very slow
You'd think he had no place to go!

So now they come upon a bridge
And on the other side -- a fridge!
Well -- you know men -- they've gotta see
What's inside the fridge to eat!
One flashlight is the light they've got
To guide them to the eating spot.
The batteries will only last
Seventeen minutes -- that's a fact.
The bridge, alas, -- and here's the trap --
Is apparently a piece of crap.

So only two men at a time
can cross the bridge -- or they'll sink in brine!
How can they all then make the trip?
And use the light so no one slips?
Send the fast guys first across
The fastest returns with little loss.
The pokey ones are next to go
While Fast Guy waits (they sure are slow)
Then send the other fast guy back
To get his friend and complete the pack.

And this rounds out, for all to see
Another FAQ in poetry.