Logic Puzzles

Bridge and Torch

There are 4 men who want to cross a bridge. They all begin on the same side. You have 17 minutes to get all of them across to the other side. It is night. There is one flashlight. A maximum of two people can cross the bridge at one time. Any party who crosses, either 1 or 2 people, must have the flashlight with them. The flashlight must be walked back and forth, it cannot be thrown, etc. Each man walks at a different speed. A pair must walk together at the slower man's speed.

Man 1 takes 1 minute to cross
Man 2 takes 2 minutes to cross
Man 3 takes 5 minutes to cross
Man 4 takes 10 minutes to cross

For example:

If Man 1 and Man 4 walk across first, 10 minutes have elapsed when they get to the other side of the bridge. If Man 4 returns with the flashlight, a total of 20 minutes have passed and you have failed the mission.

Knights and Knaves

A very special island is inhabited only by knights and knaves. Knights always tell the truth and knaves always lie.

You are a visitor on this island and came upon a knight and a knave, but do not know which is which. You are allowed to only ask one yes or no question to determine who is a knight and who is a knave.

What yes or no question would you ask them?

Hardest logic puzzle ever (supposedly)

Three gods A, B, and C are called, in no particular order, True, False, and Random. True always speaks truly, False always speaks falsely, but whether Random speaks truly or falsely is a completely random manner. Your task is to determine the identities of A, B, and C by asking three yes or no questions; each question must be put to exactly one god. The gods understand English, but will answer all questions in their own language, in which the words for yes and no are da and ja, in some order. You do not know which word means which.

For clarification, a single god may be asked more than one question, questions are permitted to depend on the answers to earlier questions, and the nature of Random's response should be thought of as depending on the flip of a coin hidden in his brain. If the coin comes down heads, he speaks truly, if tails, falsely.

to Dave's Tricks & Puzzles Page