Brainteasers: My Interview Kryptonite

Sorry everyone, I know I’ve been MIA lately but I’ve been busy the past few weeks with interviews (a good thing!).

I recently had an interview with a well-known company in the Bay Area. I was doing well throughout the interview and nailing all the fit questions, behavioral questions and case questions. Then as I came down the home stretch, my interviewer said, “Before we finish the interview, let me ask you some brain teasers”. All I had to do was nail these brain teasers and I would have aced the interview. Other than that, there was no pressure at all.

Question #1:

If you drive up a mountain at 10 miles per hour, how fast do you have to drive back down in order to average 20 miles per hour over the entire trip?

The answer:

First I said 30 miles per hour because you drive up 10 miles for an hour, and drive down 30 miles in an hour = 40 miles / 2 hours = 20 miles per hour. I was pretty excited that I solved the brain teaser until the interviewer clarified that both legs must be equidistant.

Apparently the correct answer was that there is no solution. It is impossible to average 20 miles per hour over the entire trip! Apparently it was my mistake to assume that an interviewer would ask me a brain teaser with a solvable solution. Thus, I wasted all my allotted 30-60 seconds by taking the plug and chug approach. Who asks brain teasers with no solvable answers anyways?  

Question #2:

You have a 6-shot revolver and you load two bullets into the first two chambers, and then the remaining four chambers are empty. You spin the chamber and pull the trigger. The gun does not go off. You have to shoot the revolver again. You do NOT want the gun to fire. Do you pull the trigger right away or do you spin the chamber first?

The answer:

Well, I was still sweating over the fact that I did not solve question #1 so it was hard to be 100% focused on question #2. But I said that you pull the trigger right away because of my gut feeling (plus I figured I had a 1 out of 2 chance of guessing correctly). The interviewer then said, “Why? What are the probabilities for each scenario?” Unfortunately this was like finance class, and you have to show work with your answer in order to get credit. I came close to getting the right answer but ultimately I could not figure out the exact probability of pulling the trigger right away in a timely manner. No credit for me.

If you spin the chamber, you have a 2 out of 6 or 1/3 chance of firing a bullet but if you don’t spin the chamber, the probability is 1 out of 4 because the next possible chamber cannot be the chamber from the first round and it also can't be the 2^nd bullet or else the gun would have fired a bullet the first time around. Therefore, there are 4 possible total chambers and 1 bullet that could be fired if you pull the trigger right away, hence the 1 out of 4 probability.

Hopefully some of you will be asked to answer similar brain teasers in your interviews and you will be prepared to ace them! Just don’t answer too quickly or else you will look suspicious!

Comments

[Jen]

Hi! Our mutual friend, Greg Mak, sent me a link to your blog, and I just wanted to drop a comment. You are a brave soul to try and find a job in CA right now! LA is my hometown, and I still had to look elsewhere. (I'm in NYC as of now). Love your blog. I hope you continue with this. I wish you luck on your job hunt, and I look forward to your updates!

[Alexander Pan]

Jen - Thanks for checking out my blog. Geographic location makes a HUGE difference depending on your education/background. CA is a terrible job market if you have a finance background without a tech or engineering background (basically my story). NYC would probably be a much better job market for me based on my work experience/education but I really like it out here. Hopefully things will work out!

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