At a job interview, most people are prepared for the obvious questions like for example, “what attracted you to the role?” or “Why should we hire you?” But in order to get a job there, you’ll have to answer some tricky questions first. Interviews are a mix of technical questions based on your past work experience and some mind-boggling puzzles.

Tech engineers are regarded as some of the smartest people in the world. But even they have a hard time answering the brain teaser questions a lot of tech companies like to ask during job interviews. We combed through recent posts on Glassdoor to find some of the trickiest brain teaser questions they get asked, and the best way to answer them. Some require solving tricky math problems while others are simple but vague enough to keep you on your toes.

Here are some of those Tricky Questions:

1. Asked to: Software engineer at Facebook

Q: You have two lightbulbs at a 100-story building. You want to find the floor at which the bulbs will break when dropped. Find the floor using the least number of drops.

Suggested answers:

Option 1: Start moving up in increments of 10 floors and dropping the bulb until it breaks (i.e., drop from floor 10, if it doesn’t break, drop from floor 20, etc.). Once the bulb breaks, move down to the floor before it broke on and start moving up floors in increments of one until the second bulb breaks. This results in a worst case scenario of 19 drops.

Option 2: ’19 drops’ is not the best worst-case scenario. Imagine trying floor 16, if it breaks, you try 1 – 15 and that’s 16 tries. If it doesn’t break, then try floor 31 and if it breaks, then try 17 – 30 (so 16 tries, including the try on floor 16). And so on (45, 58, 70, 81, 91, 100). If you reach 91, you’ll have tried 7 floors so far and if it doesn’t break, then there are 9 more tries to get to 100 (thus 16 in the worst case.)

2. Asked to: Software Engineer at Apple

Q: You have 100 coins laying flat on a table, each with a head side and a tail side. 10 of them heads up, 90 tails up. You can’t feel, see or in any other way find out which side is up. Split the coins into two piles such that there is the same number of heads in each pile.

Suggested answers:

Take 10 coins from the pile and flip them. You will achieve the desired.

Logic:

If the 10 coins are all heads, then after flipping it becomes tails thus has zero heads, same as the left out pile.

If the 10 coins have only 1 head, then after flipping, the pile will have 9 heads. The left out pile too has 9 heads.

And so on.

3. Asked to: Systems engineer at Google:

Q: How many trailing zeros are in the number 5! (5 factorial)?

Suggested answers:

Option 1: 5! = 120. So there is 1 trailing zero.

Option 2: This sounds like one geared not so much towards getting the right answer, but getting to it the right way. If you think a bit and say “one”, the interviewer will know you did it the brute-force way, doing the math. You’d get the answer faster, and probably impress them more, if you think instead how many times a ten will be produced in doing that math, rather than what the actual result of the math will be.

4. Asked to: Software Engineer at Cisco

Q: If you have a square room with no roof, and you had four flagpoles you had to plant on the walls so that each flagpole touched two walls, how would you do it?

Suggested answer:

The answer was that by planting them on the corners, each one is touching two walls because each corner is part of two walls. I wanted to pierce two walls with a pole horizontally too. They said it was an innovative solution.

5. Asked to: Manager at Amazon

Q: If you had 5,623 participants in a tournament, how many games would need to be played to determine the winner?

Suggested answers:

Option 1: The interviewer is not looking for the right answer because there can be many. What he/she is looking for is your logical approach in solving the answer. So you could start by probing more is first I would like to understand if 5,623 participants represent the number of team or individuals. Then ask the next logical question based on the answer.

Option 2: 5,622. Assuming it is a single elimination tournament. All teams lose one game except the champs. It’s always # of teams – 1.

6. Asked to Software QA Engineer at IBM:

Q: How do you weigh an elephant without using a weighing machine?

Suggested answers:

Take a big boat or may call it a ship. Take the elephant on the boat and mark the point up to which the ship sinks after the elephant is on the ship. Now take the elephant off and put some weight like 1000 kgs or so and keep putting weights on the ship until the ship sinks to the same level as was with the elephant in it. When the same level is attained add all the weights u have put on the ship that would be the approx weight of the elephant.

7. Asked to: ASIC Verification engineer at Zoran

Q: You have 2 pieces of rope, each of which burns from one end to the other in 30 minutes (no matter which end is lit). If different pieces touch, the flame will transfer from one to the other. You cannot assume any rope properties that were not stated. Given only 1 match, can you time 45 minutes?

Suggested answers:

Take one rope (Rope A), place it down as a circle. Light match and start burning rope A at the tips that are touching. When the rope completely burns out, 15 minutes will have passed (since both ends are burning and being consumed at once). Hold the second rope (Rope B) straight and place one end so that it will immediately catch fire when the two burning points from (Rope A) finally touch and are just about to burn out. Thus, 15 minutes on Rope A + 30 minutes on Rope B gives you 45 minutes.

8. Asked to: Software Development Engineer in Test at Webtrends

Q: There are 20 different socks of two types in a drawer in a completely dark room. What is the minimum number of socks you should grab to ensure you have a matching pair?

Suggested answer:

I’m not a mathematician, statistician, or highly analytical but if you pick up 3 socks they could still be all the same type – even if the odds are 50%. Odds do not equal reality. So the only way to ‘ensure you have a matching pair’ is to pick up 11 of the 20. This is the only foolproof guaranteed way to get a pair (in the real world and not the world of odds).

9. Asked to: Software engineer at D. E. Shaw & Co.

Q: Given 9 balls all of which weigh the same except for one, what is the minimum of weighings necessary to find the ball weighs more (or less)?

Suggested answer:

You could do this with two weighings assuming it’s a two pan balance – (1) place three balls on each side – if they balance out then it’s the remaining three that has abnormal ball (2) out of that group, place one ball on each side – if balances it out, the abnormal ball is the remaining one. If the weighing in step (1) does not balance out, grab the group of three balls that is light or heavy and repeat step (2) described above.

10. Asked to: Web Technology Intern at Riot Games

Q: 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 the box it labels. 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?

Suggested answer:

Open the box that is labeled “Apples and Oranges”.

You know that since none of the labels are correct, the box must either contain only apples or only oranges.

Suppose that you remove an apple from that box. Therefore, that box is the “Apples Only” box.

One of the two remaining boxes must be the “Oranges Only” box. However, one of them is labeled “Apples Only”, and the other is labeled “Oranges Only”.

Therefore, the one labeled “Apples Only” is the box that contains only oranges, and the box labeled “Oranges Only” is the box that has both kinds of fruit.

11. Asked to: Software engineer at Raytheon

Q: In front of you are three light switches. Only one does anything, and it turns on the light downstairs. From here you can’t see the light, and it makes no sound. You must determine which switch operates the light, BUT you can only go check it once. How do you figure out which switch is for the light?

Suggested answer:

Flip any switch you want. Wait for about 5-10 minutes to let the bulb heat up. Flip that same switch off and another one on. Go check the light. If it’s off and hot, it was the first switch, if it’s on it was the second and if it’s cold and off, it was the last one.