Answering puzzles for interview questions
A hiring manager may use puzzles for interview questions to add more interest to the conversation and test a variety of skills. The best puzzles for an interview are typically those that have a clear-cut answer. However, the correct answer isn't the only thing that potential employers are looking for. Examine these examples of interview questions in puzzle format more closely, and you'll see that they can reveal a great deal about your capabilities as a potential hire.
Puzzle interview questions with sample answers
Hiring managers may use puzzles for interview questions to assess your mathematical, analytical, and reasoning skills. These questions can also help an interviewer determine whether you can describe your thought processes clearly and concisely. Below are some examples of the types of puzzles an interview may include and tips to help you nail an interview with these questions.
You have a four-minute hourglass and a seven-minute hourglass. How do you measure nine minutes?
This problem requires some basic math skills and a great deal of critical thinking. To measure nine minutes, you must use the four-minute hourglass twice to measure eight minutes. You must also start the seven-minute hourglass at the same time you start the four-minute hourglass the first time. When the seven-minute hourglass runs out seven minutes into the eight-minute period, you must flip it over immediately. When the four-minute hourglass runs out one minute later, flip the seven-minute hourglass with one minute’s worth of sand in the bottom over to count the final minute.
Example answer: To begin, I turn over both hourglasses at the same time. When the four-minute hourglass runs out, I immediately flip it over so that the next time it runs out I will know eight minutes have passed. When the seven-minute hourglass runs out, I immediately flip it to restart it. When the four-minute hourglass finishes again, eight minutes have passed and the seven-minute hourglass has one minute measured out. I flip the seven-minute hourglass again to count out the last minute, thus reaching nine minutes in total.
Four people need to cross a bridge at night. They have only one flashlight, which must be used to safely get across. The bridge can support two people at a time. Each person travels at a different speed. When two people cross at the same time, they move at the slower person’s pace. If person A takes one minute, person B takes two minutes, person C takes five minutes, and person D takes eight minutes, what’s the minimum time needed to cross the bridge?
This is a deceptive question, because you may initially want to send the fastest person back and forth for every trip. However, this isn’t the quickest solution because it separates the two slowest people into a five-minute trip and an eight-minute trip. Sending these two together reduces that time to a single eight-minute trip.
Example answer: I first send A and B across together, taking two minutes. A takes one minute to carry the flashlight back. C and D cross together for the second trip across, taking eight minutes. B then takes two minutes to return the flashlight to the other side and another two minutes to cross the final time with A. In total, it takes 15 minutes for all four people to cross as quickly as possible.
You have 12 coins that look identical. One is a counterfeit and weighs less than the others. How can you identify the counterfeit in three weighings using a two-pan balance scale without weights?
Companies like Google, Amazon, and Microsoft use this interview puzzle. It requires a great deal of critical thinking to correctly divide the coins. You must compare the coins in groups of three. If the first two sets of three coins weigh the same, move on to compare the second set of six in two groups of three. When you find a combination of three that’s lighter than the others, you will compare two coins from that group to find the counterfeit.
Example answer: For the first weighing, I compare six coins, with three on each side of the scale. If these are equal, I proceed to the second weighing and compare the second six coins in the same manner. When I find a grouping of three that weighs less, I’ll know that the counterfeit is in that group. I can then weigh any two of these three coins to find the counterfeit. If one of the two is lighter, that’s the counterfeit. If the two I select are equal, the third coin is the counterfeit. Thus, I can determine which coin is counterfeit in two or three weighings.
There are three lights inside a closed room and three corresponding switches outside the door. You can switch the lights on and off as many times as you’d like, but you must leave them in place once you open the door. How will you determine which switch corresponds to which light?
This might seem like a math problem at first glance, but these types of puzzle questions for an interview are actually testing your logic and reasoning. It’s not enough to check which lights are on or off. You must also consider how you can tell if a light was on previously.
Example answer: I would turn the first switch on, leave the second switch off, and turn the third switch on and off several times before leaving it off. When I open the door, the first switch goes to the light that’s on, the second switch goes to the light that’s off and cold, and the third switch operated the bulb that’s off and warm.
How can you place 10 coconuts in five straight lines such that each line has four coconuts?
You might encounter this question with bowling balls, blocks, or another type of object. Whatever form these puzzle questions for interviews take, they’re designed to examine your spatial reasoning.
Example answer: I arrange the coconuts in the shape of a five-point star with four coconuts in each line. Each coconut is used in two of the five lines.
There are 100 doors that you will pass 100 times. On the first pass, you open every door. On the second pass, you close every second door. On the third pass you attend to every third door, and so forth, closing the open doors and opening those that are closed. After the 100th pass, which are open and which are closed?
The 100 doors problem is popular with those using interview puzzles to test their candidates. You can solve this problem with a math equation. The number of times you would open or close each door corresponds with the number of divisors that door has. For example, the divisors for door 38 are one, two, nineteen, and thirty-eight. Explaining your reasoning is particularly important with interview puzzle questions like this. While you could answer it correctly with a very slow and laborious drawing, you can impress your interviewer far more by offering a simplified mathematical process for reaching the correct answer.
Example answer: To solve this problem, I would determine the number of divisors each door has. The doors with an even number of divisors are closed, and the doors with an odd number of divisors are open.
You have three boxes. One box contains two white balls, one has two black balls, and one has one white ball and one black ball. All three boxes are labeled incorrectly. You may draw only one ball from one box. Which box do you draw from to fix the labels on all three boxes?
This logic puzzle is simpler than it seems. As with most puzzles for interviews, there are different ways to ask this. Your interviewers may ask how many of the boxes you need to draw a ball from to fix the labels, in which case the answer is one. As always, it’s best to explain your logic.
Example answer: I would draw a ball from the box labeled BW. Since the label is wrong, this box must contain two of whatever color ball I draw. I would then move either the WW or BB label to this box accordingly. The box that I remove it from must have two of the other color, since all the labels are wrong. Thus, the final box has one black and one white.
There is a bag with 20 blue balls and 13 red balls. You remove balls two at a time. If they’re the same, you replace them with a blue ball. If they’re different, you replace them with a red ball. What color is the last ball?
This puzzle interview question comes down to one simple fact. You have an even number of blue balls and an odd number of red balls. Since you essentially put the red ball back any time you draw a red and blue, you will always have at least one red ball in the mix.
Example answer: The last ball is red because the bag has an odd number of red balls, and you replace this ball any time it’s not drawn with a matching pair.
Reading through popular interview questions is a great way to prepare and refine your responses. Reviewing common puzzles for interviews will help you prepare for these tricky questions.
Learn more: Review common interview questions for thousands of job titles to make sure you’re prepared for anything you might encounter in your job search.