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The Best Interview Question Ever

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Q: What is the best interview question ever?

A: There are two questions that I loved asking to quickly assess candidates:

The first question is for anyone interviewing for any position (or my kids) or just for fun. The second question is Einstein's Riddle and the one I loved to ask programmers, developers, strategy consultants, etc - in other words, jobs that required structured thinking and more mental horsepower.

Question #1 - Crossing the Bridge:

If the goal is to drive across a one-mile bridge in one minute – and you’ve already driven across the first half of the bridge at the speed of 30 MPH – how fast do you need to drive across the second half of the bridge in order to make your goal of one minute?

Typical Answers:

  • Most people quickly and confidently answer 90 MPH because then the average speed is 60 MPH. They, of course are wrong.
  • Some people will more carefully consider it and really struggle, but give up out of the potential embarrassment of perhaps getting it wrong.
  • Only 5% or so will answer it correctly (in my experience and my recollection, but I haven't kept careful track of that).

The Correct Answer: You can't make the goal. You've already spent the one minute only going half of the distance, so there is no way you can still cross the bridge in your goal of one minute because no more time remains.

Why it is Difficult to Answer: The issue is that most people trying the answer the question don't properly do the units conversion of Miles per Hour to Miles per Minute because the question sounds deceptively simple. If they converted, they would quickly understand that 30 Miles/Hour means 0.5 miles per minute and realize that one full minute has already elapsed (30 Miles/Hour x 1 Hour/60 Minutes = 0.5 Miles/Minute). I've received a lot of blank stares even when I explain the answer and then I know that the interview is over, because if they cannot even follow the explanation, I'm just not going to have the patience to work with them.

Question #2 - Einstein's Riddle:

Here's the riddle in three pieces:

Background:

  • There are 5 houses in 5 different colors
  • A person with a different nationality lives in each house
  • The 5 owners drink a certain type of beverage, smoke a certain brand of cigar, and keep a certain pet
  • No owners have the same pet, smoke the same brand of cigar, or drink the same beverage

Clues:

  • The Brit lives in the red house
  • The Swede keeps dogs as pets
  • The Dane drinks tea
  • The green house is on the left of the white house
  • The green homeowner drinks coffee
  • The person who smokes Pall Mall rears birds
  • The owner of the yellow house smokes Dunhill
  • The man living in the center house drinks milk
  • The Norwegian lives in the first house
  • The man who smokes Blend lives next to the one who keeps cats
  • The man who keeps the horse lives next 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

The Question: Who has fish for a pet? (I would then give them the Background, Clues, and Question in writing and offer them a whiteboard marker and ask them to show me their work.)

Typical Answers:

Einstein created this riddle and estimated that only 2% of the world's population could properly answer it. Most people completely give up in exasperation and don't even attempt to answer it because they either think it is impossible to answer, or they can't seem to figure out that you need to start digging in and nail down one detail at a time.
The few candidates that are really, really good with frameworks and logical processing, will write out the grid (the axis is often reversed, but doesn't matter, as long at it is a grid) and they will continue to loop through the clues until they've filled in the grid. The reason for the numerous loops is that many of the clues are not useful until you've filled in what you already know.

The Best Candidate I Had: I had one 20-year old project manager interview candidate go to my whiteboard and answer it in about 3 minutes flat. I was stunned. Not only did I hire him on the spot, but within 18 months he was the Managing Director of my entire European operation (and couldn't even rent a car yet). By 22 years old he was at Stanford Business School getting his MBA (I wrote a letter of recommendation, so I get some credit, right?). He was truly in the top 2% of anyone I've ever met or worked with. (He grew a beard to look older because he feared that our European clients would never want to work with a 21 year-old project manager or managing director.)

Tips on How to Answer: The easiest way to solve this puzzle is to start with a grid like this and begin filling it in with clues:

Best Interview Question 1.png

Clues 8 and 9 allow us to fill in cells directly:

Best Interview Question 2.png

Clue 14 places the blue house next to the Norwegian, and the green and white houses as 4 and 5 (not 3 and 4 because clue 5 has the green house owner drinking coffee):

Best Interview Question 3.png

Clue 1 says that the Brit lives in a red house, which rules out all but 3. By process of elimination, the first house is yellow. Clue 7 adds to this the fact that the owner smokes Dunhills:

Best Interview Question 4.png

Clue 11 places horses next to the Dunhill house. The beer drinker smoke Blue Master, the Dane drinks tea, and Milk and Coffee are in houses 3 and 4. So the Norwegian, who smokes Dunhill, drinks water. And the water drinker's neighbor smokes blend. With the only beverages remaining being tea and beer, and knowing that the beer drinker smokes Blue Master, the second house has the tea drinking Dane:

Best Interview Question 5.png

The last beverage, beer, goes in house 5 with it's Blue Masters. The German Prince-smoker must, therefore, be in House 4. This leaves the Swedish dog owner for House 5, then the bird-rearing Pall Mall smoker for House 3:

Best Interview Question 6.png

Since the cat owner lived next to the Blend smoker, we can see that there is only one slot left, in the German's house. So it is the German who keeps the fish:

Best Interview Question 7.png

If you have a candidate that can solve this, you need to hire them!

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