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.

19 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

  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.

  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. 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….

  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

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