Dave's Puzzles Page

Puzzles Page

12 Coin Puzzle

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 other.
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.

2-Man Bridge and 1 Flashlight Puzzle

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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