Brainteasers

Here are some brainteasers I've found and enjoyed. Hopefully you'll like them as well.

Rustan Leino

I found these brainteasers on my colleague Rustan Leino's page. He hadn't found time to post the answers, so I put them here.

Knight, Knave, Commoner #1 Knight, Knave, Commoner #2 The Hidden Card #1 The Hidden Card #2 The Electrician Problem Points on a Circle Stabilizing Nodes from an Anchor Table with Four Coins The Prisoners and the Switch Line of People with Hats #1 Line of People with Hats #2 Three Hat Colors Multiple Hat Colors Flipping Cards Burning Ropes to Measure Time Boris and Natasha Determining a Hidden Digit Placing Coins on a Table The Worm and the Rubber Band Right Triangle with a Given Side Summing Pairs of Numbers to Primes The Exact Batting Average Passing Alternating Numbers of Coins A Special Squarish Age Psycho Killer Mixing Vinegar and Oil Path on the Surface of the Earth Fair Soccer Championship Cutting Cheese Finding a Counterfeit Coin Children and Light Switches Subsequence of Coin Tosses Voting on Coin Distribution Determining the Number of One Hat Coins in a Line Multiples in the Fibonacci Series Mixed-up Airplane Seats Colored Balls in Boxes Catching a Spy Making a Square Larger Frugal Selection of Weights Opening Boxes in a Courtyard Finding a Restaurant in a Park Dropping 9-Terms from the Harmonic Series Poisoned Wine Lemmings on a Ledge Witches at a Coffee Shop Finding a Hermit Genders of Children Three-Person Duel Capturing a Pirate Ship Ages of Children Dropping Eggs The Duck and the Fox Guessing Each Others' Coins Weighing Piles of Coins Digit Sums of Multiples of 11 Alternating T-shirt Colors Chomp Rectifying a Pill Mistake Handshakes at a Dinner Party Car and Key Hide-and-Seek Transporting Bananas Planar Configuration of Line Segments Enclosing Land by Fence Pieces Picking the Larger of Two Cards Evaluating a Polynomial A Game of 2009 Cards