Dave's Puzzles Page
Given 12 coins identical in every way except one: one of them weighs
either more or less than the remaining eleven.
Using a simple balance (not a scale) exactly three times, identify
which of the twelve coins is the odd coin, and whether it is heavier
or lighter than the rest.
Note that a balance only tells you that one side is heavier than the
This is a rather difficult problem, but
here's a hint.
I think there's only one solution, so
here's how I solved it.
There are four people who need to cross a 2-man bridge at night, but
the bridge is long, and there's only one flashlight, and the bridge is
old and the flashlight needs to be used the whole way. The people in
the group vary in their confidence in crossing the bridge (the girls
take longer), and the amount of time it takes them to cross is known
to be: 10 minutes, 5 minutes, 2 minutes, and 1 minute.
Devise a way to get them all across in the least amount of time.
Here's a hint.
Here's the solution.
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