Friday Puzzle

Puzzles presented and solved at lunch break every Friday, by SEN3, CWI


  • Alexandra Silva
  • Christian Koehler
  • Clemens Kupke
  • Dave Clarke
  • David Costa
  • Farhad Arbab
  • Frank de Boer
  • Helle Hansen
  • Jan Rutten
  • Jose Proenca
  • Lacramioara Astefanoaei
  • Mohammad Mahdi Jaghouri
  • Stephanie Kemper
  • Sun Meng
  • Tom Cothia
  • Young-Joo Moon


It is now a blog! Go to:


March 2007

Product and Sum (still unsolved)

(Seen in a presentation by Rineke Verbrugge, in NVTI Theory day, 2007)

Of two unknown integers, each between 2 and 99 inclusive, a person P is told the product and a person S is told the sum. When asked whether they know the two numbers, the following dialog ensues:

P:  "I don't know them."
S:  "I knew that already."
P:  "Then I now know the two numbers."
S:  "Then I now know them, too."

What are the two numbers?  Prove that your solution is unique.

1000 bottles of wine and 10 rats

(Can be found in Leino's puzzle page)

A mad scientist has 1000 bottles of whine, but one of them is poisoned. He also has 10 rats, for which the poison bottle will kill in any amount, within 10 days.

The scientist wants to have a party on the 11th day, and remove the poisoned bottle until there. How can he find the right bottle?

February 2007

2 pairs of gloves for 3 doctors

(Given by Lacramioara)

Three surgeons have to operate on a patient, one at a time. But there are only two par of latex gloves, and none of the doctors wants to touch the blood of the patient. Consider that each time a surgeon removes a pair of gloves it stays inside out.

Psycho killer

(Can be found in Leino's puzzle page)

A building has 16 rooms, arranged in a 4x4 grid. There is a door between every pair of adjacent rooms ("adjacent" meaning north, south, west, and east, but no diagonals). Only the room in the northeast corner has a door that leads out of the building.

In the initial configuration, there is one person in each room. The person in the southwest corner is a psycho killer. The psycho killer has the following traits: If he enters a room where there is another person, he immediately kills that person . But he also cannot stand the site of blood, so he will not enter any room where there is a dead person.

As it happened, from that initial configuration, the psycho killer managed to get out of the building after killing all the other 15 people. What path did he take?

Older puzzles

The Fake coin

(Can be found in Leino's puzzle page)

You have 12 coins, 11 of which are the same weight and one counterfeit coin which has a different weight from the others. You have a balance that in each weighing tells you whether the two sides are of equal weight, or which side weighs more. How many weighings do you need to determine: which is the counterfeit coin, and whether it weighs more or less than the other coins. How?

The electrician problem

(Can be found in Leino's puzzle page)

You're an electrician working at a mountain. There are N wires running from one side of the mountain to the other. The problem is that the wires are not labeled, so you just see N wire ends on each side of the mountain. Your job is to match these ends (say, by labeling the two ends of each wire in the same way).

In order to figure out the matching, you can twist together wire ends, thus electrically connecting the wires. You can twist as many wire ends as you want, into as many clusters as you want, at the side of the mountain where you happen to be at the time. You can also untwist the wire ends at the side of the mountain where you're at. You are equipped with an Ohm meter, which lets you test the connectivity of any pair of wires. (Actually, it's an abstract Ohm meter, in that it only tells you whether or not two things are connected, not the exact resistance.)

You are not charged for twisting, untwisting, and using the Ohm meter. You are only charged for each helicopter ride you make from one side of the mountain to the other. What is the best way to match the wires? (Oh, N>2, for there is no solution when N=2.)

Fair division among N people

(Given by Mahdi)

You have some amount of gold dust, and you want to share it among N people such that every person agrees to have exactly 1/N of the gold. The main problem is that there is no measurement cup to help dividing.

This problem is well known for 2 people: one divides the gold and the second chooses a half. How can this be generalized for N people?

Flipping cards

(Can be found in Leino's puzzle page)

You're given a regular deck of 52 playing cards. In the pile you're given, 13 cards face up and the rest face down. You are to separate the given cards into two piles, such that the number of face-up cards in each pile is the same. In separating the cards, you're allowed to flip cards over. The catch: you have to do this in a dark room where you cannot determine whether a card is face up or face down.

The breaking crystal balls

(Given by Tom)

You have two equal crystal balls for which we know that when dropped from a building with 100 floors will break. You just don't know which is the first floor from where the balls will break.

To find out the floor you can go to the first floor, drop one of the balls, and if it doesn't break you go to the second floor and try again, until the limit is found. Having 2 balls, how can you minimize the maximal number of tries until the limit is found?

Burning ropes to measure time

(Can be found in Leino's puzzle page)

You are given a box of matches and a piece of rope. The rope burns at the rate of one rope per hour, but it may not burn uniformly. For example, if you light the rope at one end, it will take exactly 60 minutes before the entire rope has burnt up, but it may be that the first 1/10 of the rope takes 50 minutes to burn and that the remaining 9/10 of the rope takes only 10 minutes to burn. How can you measure a period of exactly 45 minutes? You can choose the starting time. More precisely, given the matches and the rope, you are to say the words "start" and "done" exactly 45 minutes apart.

Find the light bulb

(Given by Dave)

In the ground floor you have three switches, each for a lamp in the first floor. Initially all the switches are turned off, but you don't know which switch corresponds to which light bulb. How can you find the correct matching between switches and light bulbs by changing the switches and go upstairs only once?