Russian roulette interview puzzle

"Let's play a game of Russian roulette," begins one interview stunt that is going the rounds at Wall Street investment banks. "You are tied to your chair and can't get up. Here's a gun. Here's the barrel of the gun, six chambers, all empty. Now watch me as I put two bullets in the gun. See how I put them in two adjacent chambers? I close the barrel and spin it. I put the gun to your head and pull the trigger. Click. You're still alive. Lucky you! Now, before we discuss your résumé", I'm going to pull the trigger one more time. Which would you prefer, that I spin the barrel first, or that I just pull the trigger?" 

The good news is that the gun is imaginary. It's an "air gun," and the interviewer makes the appropriate gestures of spinning the barrel and pulling the trigger. The bad news is that your career future is being decided by someone who plays with imaginary guns. This question is a logic puzzle. It has a correct answer, and the interviewer knows what it is. You had better supply the right answer if you want the job. 

In the context of a job interview, solving a puzzle like this is probably as much about stress management as deductive logic. The Russian roulette question exemplifies the mind-set of these interviews - that people who can solve puzzles under stress make better employees than those who can't. The popularity of today's stress- and puzzle-intensive interviews is generally attributed to one of America's most successful and ambivalently regarded corporations, Microsoft.

Solution:

The spin-the-barrel option is the simpler of the two to analyze.
There are two bullets in six chambers, or, to put it more optimistically, four empty chambers out of six.

Spin the barrel, and you've got a four-in-six, or two-in-three, chance of survival.

For the other option, look at it this way. The four empty chambers are all contiguous. One of them just spared your life. For three of these four empty chambers, the "next" chamber in succession will also be empty. The remaining empty chamber is right before one of the two bullets. That means you have a three-in-four chance of survival when you don't spin.

Three-fourths is better than two-thirds, so you definitely don't want the barrel spun again.

Source:

"How Would You Move Mount Fuji?"

Microsoft's Cult Of The Puzzle
By William Poundstone