Logic Puzzles
Break out Session and Resources
Ph.D. Training – Published 03.21.2008
Piled Higher and Deeper by Jorge Cham
www.phdcomics.com
Interviewers know if you’ve seen problems before.
They're interested in seeing your thought process – not memorization skills
Agenda
Four Logic Puzzles
Resources
Ground Rules
 If you know the solution already don’t say it – instead try to
think of the process you would use to solve to problem
 Entertain all thought processes – (even if you know it’s
wrong)
 If you were perfect in your thought process these exercises
would be pointless
 Keep some notes on each problem so we can compile them
and share
17 Minute Bridge Problem
Common, shows up in many forms, even a games devoted to it
The Rope Bridge
 Four people need to cross a rickety rope bridge to get back to their
camp at night.
 One flashlight and it only has enough light left for seventeen
minutes.
 The bridge is too dangerous to cross without a flashlight, and it’s
only strong enough to support two people at any given time.
 Each of the campers walks at a different speed.
 Campers{w,x,y,z} = {1,2,5,10}mins
 How do the campers make it across in 17 minutes?
Thought Process (Attacking the problem)
Solution (Extra Resources)
 http://www.novelgames.com/flashgames/game.php?id=55
 30 minute version – more people
1. (w) & (x) Cross with Torch (2 min) 
2. (w) Returns with Torch (1 min) 
3. (y) & (z) Cross with Torch (10 min) 
4. (x) Returns with Torch (2 min) 
5. (w) & (x) Cross with Torch (2 min) 
 Total 17 min
Dealing with Numbers
Estimation and Precision
 Problem 1 (Estimation)
 How many golf balls can fit in a school bus?
 Problem 2 (Precision)
 Lay 2 quarters on a flat surface so that their edges are in
contact. Hold one still and roll the other one around it. How
many rotations does the moving quarter make?
Thought Process (Attacking the problem)
Solutions
 One of many solutions:
 Assume Bus is
 50 balls tall
 50 balls wide
 200 balls long
 50x50x200 = 500K balls
 Quarters:
 The intuitive straight line answer is 1
 e.g. if the quarter rolled along the same linear distance as the
circumference of the other quarter the answer is 1
 The correct answer (due to the rotation around the original quarter)
is 2
Association
Seen in interviews, LSAT, MCAT tests etc.
There are 5 houses in 5 different colors. In each house lives a person of a different
nationality. The 5 owners drink a certain type of beverage, smoke a certain brand of
cigar, and keep a certain pet. Using the clues below can you determine who owns
the fish?
The Brit lives in a red house.
The Swede keeps dogs as pets.
The Dane drinks tea.
The green house is on the immediate left of the white house.
The green house owner drinks coffee.
The person who smokes Pall Mall rears birds.
The owner of the yellow house smokes Dunhill.
The man living in the house right in the middle drinks milk.
The Norwegian lives in the first house.
The man who smokes Blend lives next door to the one who keeps cats.
The man who keeps horses lives next door to the man who smokes Dunhill.
The owner who smokes Blue Master drinks beer.
The German smokes Prince.
The Norwegian lives next to the blue house.
The man who smokes Blend has a neighbor who drinks water.
Thought Process (Attacking the problem)
THE GERMAN OWNS THE FISH
Nationality
Norweg
Dane
Brit
German
Swede
Color
Yellow
Blue
Red
Green
White
Beverage
Water
Tea
Milk
Coffee
Beer
Smokes
Dunhill
Blend
Pall Mall
Prince
Blue
Master
Pet
Cats
Horses
Birds
Fish
Dogs
MISC
 Change the position of just one of the words below so that all
the words are in an alphabetical sequence:
 llama phoenix hyena alligator beaver elephant tortoise antelope
 A ship is docked in the harbor. Over the side hangs a rope
ladder with rungs a foot apart. The tide rises at a rate of 9
inches per hour. At the end of six hours, how much of the
rope ladder will still remain above water, assuming that 9 feet
were above the water when the tide began to rise?
Thought Process (Attacking the problem)
THINK ABOUT THE PROBLEM
 Antelope: move it from the end to the beginning, the initial
letters will then spell ALPHABET.
 Still 9 feet because the ladder will rise with the ship!
 These puzzles taken from brainbashers.com
Resources
Online Resources

http://www.techinterview.org/

http://tiny.cc/BNao2


(Handling Brainteasers and Logic Questions in an Interview)
http://tiny.cc/EM2vw

10 steps to solving any sizing question

http://www.brainbashers.com/puzzles.asp

http://www.pzzls.com/

http://www.puzzle.dse.nl/logical/index_us.html

http://brainden.com/logic-puzzles.htm

http://www.puzzles.com/projects/LogicProblems.html
Audience
 What brainteasers have you heard are common?