Top 10 Interview Puzzles

Puzzles, riddles, logical questions, and lateral thinking questions are important part of any selection process for a job interview.
You can check how good you are by answering these top 10 interview puzzles.

1. Ant and Triangle Problem

Three ants are sitting at the three corners of an equilateral triangle. Each ant starts randomly picks a direction and starts to move along the edge of the triangle. What is the probability that none of the ants collide?

2. Crossing the Bridge Puzzle

Four people need to cross a rickety bridge at night. Unfortunately, they have only one torch and the bridge is too dangerous to cross without one. The bridge is only strong enough to support two people at a time. Not all people take the same time to cross the bridge. Times for each person: 1 min, 2 mins, 7 mins and 10 mins. What is the shortest time needed for all four of them to cross the bridge?

3. Burning Rope Problem

A man has two ropes of varying thickness (Those two ropes are not identical, they aren’t the same density nor the same length nor the same width). Each rope burns in 60 minutes. He actually wants to measure 45 mins. How can he measure 45 mins using only these two ropes.
He can’t cut the one rope in half because the ropes are non-homogeneous and he can’t be sure how long it will burn.

4. Heaven’s Gate Probelm

You are standing before two doors. One of the path leads to heaven and the other one leads to hell. There are two guardians, one by each door. You know one of them always tells the truth and the other always lies, but you don’t know who is the honest one and who is the liar. You can only ask one question to one of them in order to find the way to heaven. What is the question?

5. 10 Coins Puzzle

You are blindfolded and 10 coins are place in front of you on table. You are allowed to touch the coins, but can’t tell which way up they are by feel. You are told that there are 5 coins head up, and 5 coins tails up but not which ones are which. How do you make two piles of coins each with the same number of heads up? You can flip the coins any number of times.

6. King and Wind Bottles Puzzle

A bad king has a cellar of 1000 bottles of delightful and very expensive wine. A neighboring queen plots to kill the bad king and sends a servant to poison the wine. Fortunately (or say unfortunately) the bad king’s guards catch the servant after he has only poisoned one bottle. Alas, the guards don’t know which bottle but know that the poison is so strong that even if diluted 100,000 times it would still kill the king. Furthermore, it takes one month to have an effect. The bad king decides he will get some of the prisoners in his vast dungeons to drink the wine. Being a clever bad king he knows he needs to murder no more than 10 prisoners – believing he can fob off such a low death rate – and will still be able to drink the rest of the wine (999 bottles) at his anniversary party in 5 weeks time. Explain what is in mind of the king, how will he be able to do so ?

7. Mislabeled Jar Puzzle

You have 3 jars that are all mislabeled. One jar contains Apple, another contains Oranges and the third jar contains a mixture of both Apple and Oranges.
You are allowed to pick as many fruits as you want from each jar to fix the labels on the jars. What is the minimum number of fruits that you have to pick and from which jars to correctly label them?

8. Red and Blue marbles Puzzle

You have two jars, 50 red marbles and 50 blue marbles. You need to place all the marbles into the jars such that when you blindly pick one marble out of one jar, you maximize the chances that it will be red. When picking, you’ll first randomly pick a jar, and then randomly pick a marble out of that jar. You can arrange the marbles however you like, but each marble must be in a jar.

9. Gold Bar Problem

You’ve got someone working for you for seven days and a gold bar to pay him. The gold bar is segmented into seven connected pieces. You must give them a piece of gold at the end of every day. What and where are the fewest number of cuts to the bar of gold that will allow you to pay him 1/7th each day?

10. 100 Doors Puzzle

You have 100 doors in a row that are all initially closed. you make 100 passes by the doors starting with the first door every time. the first time through you visit every door and toggle the door (if the door is closed, you open it, if its open, you close it). the second time you only visit every 2nd door (door #2, #4, #6). the third time, every 3rd door (door #3, #6, #9), ec, until you only visit the 100th door. What state are the doors in after the last pass? Which are open which are closed?

 

Puzzles are good way to exercise your brain. We will publish more such puzzles in future, so stay tuned. If you have come across any other puzzle in interviews, please feel free to share them through comments and let us try and solve them as a community.

59 Thoughts on “Top 10 Interview Puzzles

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  2. Good collection of puzzles. i was searching for a clue for Question 4 from a long time

  3. good puzzles..very helpful for puzzle preparation…….

  4. Ankur Agarwal on July 22, 2015 at 11:42 am said:

    Hi, I wanted to know, how important is puzzles for company placements? Moreover, is it always necessary that if a candidate solves a puzzle then only he will be recruited or by any chance there is still a possibility that the candidate who did not come to an answer might still be selected based on the approach that he has taken.

  5. 3. ans=>

    fold one rope 3 time i.e 60 will become=>30=>15
    and comarativelty burn other till it gets down to the size of folded rope

    • Sadly that doesn’t work. The questions states that the only constraint the ropes are under is when lit from one end it will takes 60 min to burn. Your solution would work with a normal rope with a uniform thickness, however the problem states this ropes is non-homogeneous, meaning it varies in thickness and it is impossible to know where to fold it at, since different segments will burn faster than others.

  6. 4. ask one of them wether who will return a wrong comment if queried about the right path of heaven ??

    • better would be to ask his/her gender. the liar would lie, and so we can get to know who is a liar and who is not.

      • Anirban on January 23, 2016 at 2:09 am said:

        Interesting answer. By this you can know which one has lied and which one hasn’t, but you won’t know which door leads to heaven and which one doesn’t. And you will have used up your one remaining question.

        • subodh bende on August 2, 2016 at 8:04 pm said:

          ask one of them -
          If I ask to the other one about the way of hell what ans he will give me?

          so, in both cases you will get your conclusion.

          • Just ask one “If I asked the other guardian if the right gate was hell, what would he say?” If he was the liar, he would say either say yes or no. If he says yes, that means that other one would say yes. Same basic process for all situations.

  7. 5. I would flip each coin on its side and create 2 piles of 5 coins each. This way I have both the piles with 0 coin facing heads up.

  8. Soujanya on September 25, 2015 at 11:27 pm said:

    Initially burn the first rope from both the ends and the second rope from only one end then u can calculate 30 min wen the first rope gets entirely burnt thenu continue burning of second rope from both the ends it takes 15 minutes in this way we can calculate 45 minutes

  9. Saurabh Malve on January 4, 2016 at 8:06 pm said:

    Ants prob : 2/6=0.33

  10. Prince Sunil Kumar on March 3, 2016 at 9:45 pm said:

    Q. 3 Burning Rope Problem
    1. Burn 1st rope from both end and 2nd from one end.
    2. The 1st rope will burn in 30 min then start burning of 2nd rope from 2nd end.
    3. Total Time 30+15=45
    4. Job Done….

    • thank you .your explaination was very nice.

    • Eo Raptor on August 8, 2017 at 12:02 am said:

      Burning rope.
      If I’m reading the problem correctly, each rope burns for 60 minutes. So, as someone mentioned above, take either rope, fold in half, then fold in half again. Now, cut off one of the end lines. That’s 1/4 of 60 minutes, so the remainder should burn for 45 minutes.

      Easy peasy, so what am I missing?

  11. PLZ give answer for each question . it help to user to verify their answer either ans is write or not

  12. 2.ans=>
    shortest time 10 sec

  13. rowdyraj on October 7, 2016 at 10:40 am said:

    Ans for questions 8 is
    Take 25 marvel from each jar and interchange them so that each jar contains 1/2 probability of red marvel…

  14. Mona Singh on October 16, 2016 at 6:01 pm said:

    Really Helpful! Looking for more quality posts…

  15. Naveen Kothiyal on March 7, 2017 at 1:09 pm said:

    Q.No. 6: I pull any 5 coins from the 10 coins, and flip all that pulled coins, then I get the same number of heads in bothe the sets.

  16. Shiv on April 9, 2017 at 3:35 pm said:

    Click on green line u will get solution for problem

  17. tiya on May 16, 2017 at 10:41 am said:

    4. ask the guard are you a guard?the lier wud say no since he ALWAYS lies .and the honest one wud say yes since he always tells the truth.

  18. Pingback: Secret to Solving Puzzles in Interviews – UnsaidTalks

  19. M Salim on July 30, 2017 at 1:06 pm said:

    Some of the puzzles are not clear enough to solve them correctly. When one goes to the solution then he realises, “Ohh, this also was a condition!”. Example:

    “Red and Blue marbles puzzle” : “You can arrange the marbles however you like, but each marble must be in a jar”. Looks like one is allowed to put all the red marbles on top and blue ones at bottom.

    “Gold bar problem”: it is never mentioned that you can take the gold back from the worker. Not even a hint that the worker is not spending the gold.

  20. Hey i have one question about truck tyres. One truck have 5rows of tyres and each row have 4tyres. But total tyres 10.how it is possible.Give me answer

  21. Everything is vеry open witһ a very clear clarification of the issues.
    It was really informative. Your website is extremely
    helpful. Ꭲhank you for sharing!

  22. Sanjeev on October 11, 2017 at 2:07 am said:

    Walking and listening to “Highway To hell – AC DC”. Suddenly, you see two doors with doorkeepers and the confusion of heaven & hell, truth & lie. You ask “If I ask the other guard about which side leads to heaven, what would he answer?” You get “hell”. Now you walk and play “Stairway To Heaven – Led Zeppelin”.

  23. Great puzzles. But the number has been the same for few years now. Try to update more puzzles. and there is a spelling mistake in King and wine puzzle its spelled as wind

    • aditya on August 14, 2019 at 5:59 pm said:

      in this puzzle, of king and wine, king must give a lil wine to the 10 prisoner every 1.5 hours and label the wine it prisoner and time and day , this must be done untill all 1000 bottles have been tested withing a week so within the 4th and 5th week the prisoners death of time and date will show which wine was poisoned

  24. Shreyas on November 23, 2017 at 7:35 pm said:

    21 mins
    If 1min person holds torch , he takes 2min person along with him wch takes 2mins, while returning it takes 1min for him wch gets sum up to 3 mins… And so on… At the end at 10 mins person it takes 21mins totally.

    Pls reply for the answer

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  26. Hayshitha on January 27, 2018 at 4:18 pm said:

    My question is – a plane ‘s antics on the over bridge ..
    Anyone know the answer
    Reply ….

  27. Dheeraj on March 14, 2018 at 10:03 am said:

    For bridge crossing problem.
    Total time taken by them is 17 minutes only.

    1. 1+2 will go then 1 will return: I.e 3 min
    2. 10+7 will go and 2 will return: I.e 10+2=12 min
    3. Both 1+2 will go : I.e 2 min

    If u add all 3+12+2=17. Minutes

  28. Anish Gurramkonda on March 17, 2018 at 10:17 am said:

    for questionno4:-At a time i will ask both people one question.I.e which is the way to go for heaven and hell?
    Then truth guardian will show the heaven way first and next he shows the hell.simillarly,
    hell guardian will say quite opposite to that…why becozz hell guardian wont allow the man to go heaven why means he is a lier

  29. amazing content!

  30. jagdish biradar on July 18, 2019 at 11:17 am said:

    For heaven’s gate problem
    Question is that , what is my gender ?, According to you know he lias or tells the truth

    • This way, you have used up the only chance of asking question and has not asked the very question which leads to the road to heaven.

      The better question to ask is:

      “If I ask the other gate keeper which one is the road to heaven then what will he answer?”

      The truth speaking person will give what liar will answer. The liar will lie about truth speaking person’s answer. Both ways you will get the liar’s answer, so the opposite of it is the truth speaking person’s answer and hence the door to heaven.

  31. Samatha on September 8, 2019 at 2:13 pm said:

    7 ans: if we pic third jar apple and orange, we get to know weather it is apple or orange….
    For suppose if we get apple,,, the remaining jars were orange and both of mixed I.e orange and apple.. They mentioned that the labels were mismatched, soo the jar 2 will become apple and orange mixed… And jar 1 contains orange…
    Jar1:orange
    Jar2:apple and orange
    Jar3:apple

  32. Good one ! Nice Efforts ! Thanks.

  33. Tushar on April 25, 2020 at 1:01 pm said:

    Ans for 8 is ; one jar with only one red marble and the other jar with other 99 marbles.. hence probability would be nearly 0.75

  34. MAYURESHWAR SONTAKKE on July 2, 2020 at 2:27 am said:

    Hi.

    1. 3 instances out of 2^3=8 chances that ants collies, so 5/8 probability that they won’t collide.
    2. (1) takes (2) and comes back – 3
    (10) takes (7) and (2) comes back – 12
    (2) and (1) go together – 2
    17 minutes at max.
    Key is to minimize the return time and couple time consuming journeys together.
    3. Start burning rope1 from 1 side and rope2 from both. By the time rope2 burns out, it is 30 mins and rope1 is halfway done. Now, burn the other end of rope1. By the time rope1 burns out, it is 45 mins in total.
    4. Ask anyone, what will the other guy say if I ask which door is for heaven? This is multiplicative inverse property of NOT operator in Boolean algebra.
    5. Take 5 coins from the pile and flip them all. It works because any bifurcation of coins in 5,5 bunches result in opposite combination.
    6. This is interesting. If there are 1000 prisoners, make them drink the wine. If one dies, corresponding bottle is poisonous. Another efficient answer is 30 people, out of which 3 people will die. But when we can kill 10 people, by using n(x)^(1/n) formula, where x is number of bottles and n is number of people allowed to kill, it comes out to be 20.
    7. Start with apple orange jar.
    9. 3 cuts. Cut it between 3 and 4. Place 3 on 4 pieces of bar. Cut between 2 and 3 and 5 and 6. Stack all of it one on top of another and cut it into two pieces together. It can also be solved with number of binary digits needed to represent 7, i.e. 3.
    10. For any door number, represent door number in prime format. 2^m*3^n*5^p….. (m+1)(n+1)(p+1) is the number of factors. So if above number is odd, door will be open. And if it is even, doors will be closed.

  35. 4)ask question to first one “”Are u lier”
    If he is honest he says no i dont lie but if he is a lier he would say “yes i am lier”

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  38. Deepak Kumar on April 22, 2021 at 10:13 pm said:

    Puzzles are really nice for interview purpose. You can add “Room and bulb” puzzle too.

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