Tech engineers are 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 went through Glassdoor to find some of the trickiest brain teaser questions they get asked, and the best way to answer them.
Systems engineer at Google: "How many trailing zeros are in the number 5! (5 factorial)?"
"5!=120. So there is 1 trailing zero."
"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 at 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."
Software engineer at Facebook: “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.”
"Start moving up in increments of 10 floors and dropping the bulb until it breaks (ie: 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."
"19 drops is not the best worst case scenario... imagine trying floor 16, if it breaks, you try 1 - 15 and thats 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 on and 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's 9 more tries to get to 100 (thus 16 in the worst case)."
Manager at Amazon: “If you had 5,623 participants in a tournament, how many games would need to be played to determine the winner?”
"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."
"5,622. Assuming it is a single elimination tournament. All teams lose one game except the champs. It's always # of teams - 1."
Software Development Engineer in Test at Webtrends: "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?"
"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 fool proof guaranteed way to get a pair (in the real world and not the world of odds)."
Web Technology Intern at Riot Games: “Imagine that you have three boxes, one containing two black marbles, one containing two white marbles, and the third, one black marble and one white marble. The boxes were labeled for their contents - BB, WW, BW - but someone has switched the labels so that every box is now incorrectly labeled. You are allowed to take one marble at a time out of any box, without looking inside, and by this process of sampling you are to determine the contents of all three boxes. What is the smallest number of drawings needed to do this?”
"The key thing here is that the box does not contain what it says on the label. As such you can guarantee the contents of each box with one draw. Here's how: Draw a marble from the box labeled BW (since it is labeled BW it must be either BB or WW) If you draw a white for example you know 100% that it is the WW box. Then there are only two boxes left labeled WW and BB in this example and the only two things that they can be are BB or BW. The box labeled BB cannot be BB and so hence must be the BW. This leaves the box labeled WW to be BB by elimination. Same thing works if you pick a black marble first. The key is picking from the BW box in the start and confirming what it actually is. One draw is the smallest number needed."
Software Engineer at Cisco: "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?"
"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."
Software engineer at D. E. Shaw & Co.: "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)?"
"You could do this with two weighings assuming its a two pan balance - (1) place three balls on each side - if they balance out then its 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."
ASIC Verification engineer at Zoran: "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?"
"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 mins."
"Make a T. Simple."
Software engineer at Raytheon: "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?"
"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."
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