Silicon Valley tech companies are famous for asking some pretty crazy brainteaser interview questions… I wanted to find out exactly what these questions involve. And how difficult they are to answer. So I spent a day on Glassdoor.com and a few other sites to come up with the 8 hardest and most interesting logical interview questions out there. And not just from any companies…We’re going to look at 4 tech giants known for having the toughest interviews:

Facebook

Google

Apple

LinkedIn
Time to see how many you can answer!
Facebook Brain Teaser Interview Questions and Answers:
Question 1:
A Russian gangster kidnaps you. He puts two bullets in consecutive order in an empty sixround revolver, spins it, points it at your head and shoots. *click* You’re still alive. He then asks you, “do you want me to spin it again and fire or pull the trigger again right away?” For each option, what is the probability that you’ll be shot?
Answer…
The key hint here is that the bullets were loaded adjacent to each other.
There are 4 ways to arrange the revolver with consecutive bullets so that the first shot is blank. These are the possible scenarios:
 (xBBxxx)
 (xxBBxx)
 (xxxBBx)
 (xxxxBB)
The other two scenarios would have meant you got shot on the first attempt. (BBxxxx) or (BxxxxB)
Now look at the second slot in those 4 possible scenarios above. Your odds of getting shot are 1/4 or 25%. (Only #1 would get you shot)
But if you respin… there are 2 bullets remaining and 6 total slots. 2/6 or 33%.
Question 2:
You’re about to get on a plane to Seattle. You want to know if it’s raining. You call 3 random friends who live there and ask each if it’s raining. Each friend has a 2/3 chance of telling you the truth and a 1/3 chance of messing with you by lying. All 3 friends tell you that “Yes” it is raining. What is the probability that it’s actually raining in Seattle?
Answer…
You only need 1 of your friends to be telling the truth for it to be raining in Seattle.
It’s fastest just to calculate the odds that all 3 are lying, and it’s not raining.
Each friend has a 1/3 chance of lying. Multiply the odds together… you get 1/27 (1/3 * 1/3 * 1/3).
We’re not done yet though… 1/27 is the probability that all 3 friends lied at the same time.
The probability that at least 1 told you the truth? 26/27 or around a 96% that it’s raining in Seattle.
Google Brain Teaser Interview Questions and Answers:
Question 3:
You have a 3 gallon jug and 5 gallon jug, how do you measure out exactly 4 gallons?
Answer…
We know we can’t get the final result in the 3 gallon jug. It’ll overflow. We need to end up with 4 gallons in the 5 gallon jug.
First fill the 3 gallon jug.
Then pour the 3 gallons into the 5 gallon jug.
Now the 3 gallon jug is empty, and the 5 gallon jug has 3 gallons in it.
Fill the 3 gallon jug again. Slowly pour into the 5 gallon jug. Only 2 gallons will fit because it already has 3. Now it’s full.
Exactly 1 gallon is left in the 3 gallon jug.
Dump out the 5 gallon jug.
Pour your 1 gallon into the 5 gallon jug.
Fill up the 3 gallon jug one more time and pour it into the 5 gallon jug! You have exactly 4 gallons (and possibly a job at Google)
Question 4:
Why are manhole covers round?
Answer…
Good news: If you’re tired of math questions this one will give you a break. Manhole covers are round because it’s the only shape that cannot fall through itself. The cover can never accidentally fall down the hole. Microsoft has been known to ask this question and according to Glassdoor.com, Google is asking this too now.
Apple Brain Teaser Interview Questions and Answers:
Question 5:
There are three boxes, one contains only apples, one contains only oranges, and one contains both apples and oranges. The boxes have been incorrectly labeled such that no label identifies the actual contents of its box. Opening just one box, and without looking in the box, you take out one piece of fruit. By looking at the fruit, how can you immediately label all of the boxes correctly?
Answer…
So, you know all 3 boxes are incorrectly labeled.
Go to the box labeled “Apples + Oranges.” Since the label is wrong, it must have one or the other.
This is the box to take one piece of fruit from. Whichever comes out is what that box contains. If you took out an apple, the box has only apples. If you took out an orange, vice versa.
Here’s where it gets tricky a bit tricky. But we’re almost done…
Let’s say you grabbed an apple. Move the “Apples” label over to that box. Now it’s correctly labeled.
You know the “Oranges” box is still labeled wrong (because all 3 were labeled wrong to start and you haven’t touched it). And you know it’s not “Apples”.
So it has to be “Apples + Oranges”.
The last box is “Oranges”.
The same process above would work if you had pulled out an orange at the start.
Question 6:
You have 100 coins laying flat on a table, each with a head side and a tail side. 10 of them are heads up, 90 are tails up. You can’t feel, see or in any other way find out which 10 are heads up. Your goal: split the coins into two piles so there are the same number of headsup coins in each pile.
Answer…
By pure coincidence… this is a trick my friend Mike showed me last summer. It blew my mind back then but hopefully it’ll make sense as I write it out.
You want an equal number of heads in each pile. There are currently 10 of them. You don’t know which but it doesn’t matter. All you have to do… take any 10 coins out of the 100, put them into a separate pile, and flip those 10 over.
That’s pile #1.
Pile #2 is the remaining 90 coins, unflipped. Just leave them.
You’re done. Seriously.
You can do this with any number of coins. If you had 20 coins, and 18 were heads, you’d need to take 18 of them (it doesn’t matter which) into a separate pile and flip those 18. That’s pile #1.
If you had 10 coins and 3 were heads, you’d take 3 random coins into a new pile and flip those 3 for your first pile, and the rest are your second pile.
Crazy right?
If you don’t believe me just grab some pennies and try it. There are no exceptions and it doesn’t need to be an even amount of “heads” to begin with either. It can also be zero. Or all.
LinkedIn Brain Teaser Interview Questions and Answers:
Question 7:
You’re in a room with three light switches, each of which controls one of three light bulbs in the next room. You need to determine which switch controls which bulb. All lights are off to begin, and you can’t see into one room from the other. You can inspect the other room only once. How can you find out which switches are connected to which bulbs?
Answer…
Let’s call the switches 1, 2, and 3.
Leave switch 1 off.
Turn switch 2 on for ten minutes.
Now turn it off and quickly turn on switch 3.
Go into the room and inspect…
The bulb that is still warm but not lit up is controlled by switch 2. The one that’s currently lit up is switch 3. The last one is switch 1.
Question 8:
How many golf balls would fit into a Boeing 747?
Answer…
This last one is tough, but they don’t expect you to get an accurate answer. If you get a question like this (and there are a ton of variations basketballs in a room, cellphones in Manhattan, etc.) they want to see your thought process. The hiring manager is going to look at how you work your way through it and attempt to figure it out.
If you can break a problem down into smaller pieces, stay calm, and get an answer that’s not perfect but reasonably close, you’ve done great.
They might not even know the answer. They just want to see how you approach something that’s very difficult.
On a Practical Note, What Can You Take Away From This?
Question 8 above highlights a pretty good point to remember in your interviews…
There are a lot of questions in an interview where the hiring manager values your thought process… sometimes even more than a correct answer. So if you’re stumped, talk out loud a bit and explain what you’re thinking. Ask a question if you need to. Try to break it down into smaller pieces. Specific knowledge can be taught but they can’t teach you problemsolving. That’s why they ask logical questions in a job interview, and why they ask questions where they expect you to struggle or be unsure.
If you hear questions like this, it doesn’t mean you’re doing badly. Just stay calm, walk them through your thought process, show you take a logical approach, and you’ll have a great shot at getting hired (even if you don’t come up with the perfect answer in the end!)
You Can Get Hired Even if You Give “Wrong” Answers to These Questions
Here’s a quick story: My degree is in Finance, which means I took a good amount of Accounting classes too. Early in my career, I had a phone interview for an Accounting position. To make the story short, I could not answer even the most basic accounting questions. Really simple stuff that you learn your first year in college.
Why?
It had just been too long since college and I had forgotten even the basics. And I didn’t prepare well for the interview obviously! But I tried to stumble through it and remember what I could, talking about what I was thinking. Saying things like, “well, this can’t be right because ___. So it must be related to ___.” I made some progress. But I definitely didn’t arrive at the right answer, even after three minutes of walking myself and the interviewer through it out loud.
But I still got invited to the next round in the interview process (a full day, onsite interview).
Why? Because the hiring manager liked my approach to breaking down a problem that I didn’t immediately know how to solve. That’s why being transparent and showing your thought process is one of the tips for interviews that you’ll see me say over and over. And that’s the biggest takeaway that I hope you gain from reading these brain teaser questions above (along with entertainment). You can do the same thing I did and get more job offers… even if you give a few wrong answers to difficult questions like these!
The bottom line is: Don’t panic when you get a question you don’t know; use it as an opportunity to show exactly how you work through things. Be confident with it, relax, and smile. Remember… you’re giving the hiring manager what they want! If you have interviews coming up and want to prepare further, read the top 20 interview questions here.
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I believe #1 is incorrect. I think it’s 1/5… am I missing something???
Question 3 can be done much more efficiently. All you have to do is to fill both jugs halfway. 3/2=1.5, 5/2=2.5, 1.5+2.5=4. It’s easier to estimate a half filled jug than filling a 3 gallon jug by one third. The only downside to this solution is that uncertainty is higher.
This is old: the research found brain teaser questions don’t do anything to evaluate job candidates other than stressing out the interviewee. Google has stopped using brain teasers.
Really interesting and good questions
Apologies if this has already been mentioned, but your answer to Q2 is not correct. You should be trying to solve the conditional probability problem P(Rain  YYY), and NOT 1 – P(lie, lie, lie).
The tricky part here is that you are not given all of the information you need to apply Bayes rule and solve the problem. Specifically, you additionally need to have a prior estimate of P(Rain). The interviewer/question purposefully withholds this information to see if you are able to identify how the problem should be solved, and if you can ask the right clarifying questions.
The solution, using Bayes Rule, is;
P(Rain  YYY) = P(Rain) * P(YYY  Rain) / P(YYY)
where:
P(Rain) must come from the interviewer
P(YYY) = P(Rain) * P(YYY  Rain) + P(~Rain) * P(YYY  ~Rain)
P(YYY  Rain) = (2/3)^3 # All 3 friends tell the truth when it rains
P(YYY  ~Rain) = (1/3)^3 # All 3 friends lie when it’s not raining
I hope this helps. Check out the following link for an excellent explanation and some further discussion of Bayesian vs frequentist approaches:
#3 is a correct solution but more complex than it needs to be. These companies also look for efficiency and not doubling your efforts.
Your Solution:
1. Fill up 3gal Bucket > Dump into 5gal bucket
2. Fill up 3gal Bucket > Dump 2gal into the remaining 2gal of area within the 5gal bucket
3. Empty 5gal Bucket > Dump 1gal within the 3gal bucket into the empty 5gal bucket
4. Fill up 3gal Bucket > Dump 3gal bucket completely into 5gal bucket that currently has 1gal within it.
This solution requires you to empty a bucket down the drain 1 time, fill a bucket from the faucet 3 times, and pour a bucket into a bucket 4 times.
Efficient Answer
1. Fill up 5gal bucket completely > Pour 3gal from 5gal bucket into the 3gal bucket to fill it up completely
2. Empty full 3gal bucket > Pour remaining 2gal from the 5gal bucket into the empty 3gal bucket
3. Fill up the 5gal bucket completely > Pour 1gal from the full 5gal bucket into the remaining 1gal of volume within the 3gal bucket.
This solution requires you to empty a bucket down the drain 1 time, fill a bucket from the faucet 2 times, and pour a bucket into a bucket 3 times.
If they ask you to solve the same problem, but dumping as little as possible down the drain, solution 2 is still your best bet. (1=5gal dumped, 2=3gal dumped because the problem is solved at the precise time you would need to dump it again for another step)
The only way that solution 1 is more efficient is if they ask you to solve the same problem USING as little water as possible. (1=9gal, 2=10gal)
Interesting. Thanks for the detailed reply.
The solution to Question 2 is incorrect.
You argue that the probability that at least one friend tells the truth is one minus the probability that all three friends are lying. This is correct. But you then say that we only need one friend to tell the truth for it to actually be raining. This may be true, but it takes into account scenarios where one friend is lying and the the other two are telling the truth, and vice versa. However, this is ignoring a key element: we already know that this has not happened! If all three friends are claiming that it is raining, then it is impossible that one is lying and two are telling the truth (analogously, it is impossible that two are lying and one is telling the truth) because they all said the same thing (and they know whether or not it is raining!).
One of two situations is possible: either it is raining in Seattle, or it is not. If (and only if) it is raining, then they are all telling the truth. If (and only if) it is not, then they are all lying. The probability that they are all telling the truth is 8/27; the probability that they are all lying is 1/27. The probability that they are telling the truth given that they all said the same thing is (8/27)/(1/27 + 8/27) = 8/9.
You are right. I also came to the answer 8/9 and wanted to comment that the given solution is wrong. I came across your comment and decided to just reply here :)
You’re right that the solution is incorrect, and you’re on the right track, but there’s actually not enough information, as the answer is dependent on the prior probability of it raining. If you let this prior probability be p, and apply Bayes’ rule, you should get that the probability that it’s actually raining, given that all three friends said yes, is 8p/(7p+1). If (and only if) you let p=1/2, then you recover an answer of 8/9.
This blog was… how do I say it? Relevant!! Finally I have
found something which helped me. Cheers!
Are they asking questions to everyone or just software developers?? It seems geared toward developers mostly
Hey Sam,
It’s everyone, although you’ll expert more of these if your position involves analytical thinking, logic, problem solving, etc. Many Product Managers will face these questions. Also corporate lawyers! And many others along with software developers.