Last week, I wrote an article about the most difficult job interview questions from top companies. Here are the answers to those engineering interview questions.

*The Answers to the Most Difficult Engineering Interview Questions (Source: Pinterest)*

So do you have what it takes to be a kickass engineer? These legit engineering interview questions from top companies make sure they get to hire the best engineers, the cream of the crop. Such method screens thousands of applicants easily.

*The Answers to the Most Difficult Engineering Interview Questions (Source: istock)*

And definitely, these mind-blowing questions will make you think logically and scientifically. You may need a bit of physics, statistics and some arithmetic solving them.

Have you gone nuts already? Here are the answers from last week’s article.

1.You work on the 60th floor of 100 story building. You walk into your office and find a bomb sitting on your desk. It reads 90 seconds and is counting down. What do you do? – Dropbox

*The Answers to the Most Difficult Engineering Interview Questions (Source: Dropbox)*

**ANSWER:** Since there are only 90 seconds left, removing bomb via bottom floor or roof is not possible since there is not enough time. Place bomb in corner of building so that at least half of blast is going out building away from personnel.

2. You are on a game show. There are three doors, behind one of which is a prize and the other two is a chunk of coal, and the host knows which door holds the prize. You choose door #1. Before it is opened, the host opens door #3 and reveals a lump of coal. You have the choice to stick with the door you chose originally or switch to door #2. What do you do? –Microsoft

*The Answers to the Most Difficult Engineering Interview Questions (Source: Microsoft)*

**ANSWER:** Switch doors. When you chose door #1, there was a 66% chance that the prize was not behind that door. When the host revealed the coal, there was still a 66% chance the prize was not behind the door you chose. Thus, you have doubled the odds of getting the prize by switching to door #2. The key to this puzzle is that the host knew which door has the prize.”Your choice splits the doors in two sets. Set A contains the door you selected, and the probability that is a prize behind this door is 1/3. The set B contains all remaining doors, and the probability that the winning door is somewhere in there is 2/3. By removing one door, which all have the success probability of zero because there’s coal behind them, from set B, only one door remains in B, but the overall probability for success in set B is still 2/3. Therefore you must switch.

3. Pretend 1% of the population has a disease. You have a test that determines if you have that disease, but it’s only 80% accurate and 20% of the time you get a false positive. Your test comes back positive. How likely is it you have the disease? –Google

*The Answers to the Most Difficult Engineering Interview Questions (Source: Google)*

**ANSWER:**

Fact: 1% of the population has the disease (given)

Data: Test is only 80% accurate, and 20% inaccurate (given):

Assume, Population = 10,000 people

1% have the disease = 100 people

99% do not have the disease = 9,900 people

Of the 1% who have the disease 80% tested +ve = 80

Of the 99% who don’t have the disease 20 tested +ve = 1980

To identify that you have the disease you have to test +ve and actually have the disease = 80 / (80+1980) = 80 / 2060 = 3.88%

4.If you walk 1mile south, 1mile west and 1mile north, you are exactly back where you started. Where are you? –SpaceX, (CEO Elon Musk’s Favorite Question)

*The Answers to the Most Difficult Engineering Interview Questions (Source: SpaceX)*

**ANSWER:** There are two answers, North Pole, for common people, and the South Pole, for engineers. Since South Pole presents infinite locations. If you started a mile North of the place where the circumference of the Earth is exactly a mile, then your mile long walk westward circumnavigates the Earth and the walk back takes your where you started. South Pole has circumferences that are 1/nth of a mile giving infinite solutions.

5. Design a spice rack for the blind. –Intel

*The Answers to the Most Difficult Engineering Interview Questions (Source: Intel)*

**ANSWER:** There are many answers to this but the most practical would be putting braille letters on each label for them to touch. Using smelling system may not be safe though, some spices can cause asphyxia. Another high tech solution would be integrating electronics where in the gadget tells the name of the spice once button is pressed on a certain container.

6.If you are in a boat in the middle of the pond and drop an anchor, how does the water level vary with respect to shore? –Apple

*The Answers to the Most Difficult Engineering Interview Questions (Source: Apple)*

**ANSWER**: Water level falls. By definition, the anchor, made of steel, is denser than water. Therefore, its displacement as mass, when it’s on the boat, is greater than its displacement as volume, when it’s in the water. Therefore, if you throw it overboard, the water level falls.

Now you got it like a boss.

*The Answers to the Most Difficult Engineering Interview Questions (Source: Giphy)*

## Comments 4

Sorry some are answering the wrong question.

Eg Google. It’s 80%. Because the question is you have had the test so it’s an individual risk post test. If the test was 100% accurate your risk would be 1% before the test but after the test it would 100% or 0% for having it.

Blind spice rack, the question is how would it look. Pretty because not everyone is blind, the answer given is for how would it function

First rule answer the question asked

Sorry but I don’t agree with you(referring to google’s question).I’ll use Baye’s Theorem to arrive at the same result. Let Event A = having the disease . Let A’ = not having the disease. Let Event B = Positive Result. Let Event B’ = negative result. Given, P(A) = 0.01 , P(B|A) = 0.80, P(B|A’)=0.20 , & P(A’) = 0.99. Given that the result of the test is positive, we use Baye’s Theorem to compute P(A|B) or the probability of having the disease given a positive result. Hence, P(A|B) = [P(B|A)*P(A)]/[[P(B|A)*P(A)]+[P(B|A’)*P(A’)]]

=[0.80*0.01]/[[0.80*0.01]+[0.20*0.99]]

=0.0388 or 3.88%

…The 1% was only the marginal probability but given the accuracy of the test which is only 80% prompts us to use dependent or conditional probabilities by Baye’s Theorem.

PS: i find it uneasy to input equations here so the equations are quite messy…

Thank you.

@Ian – Sorry but I don’t agree with you (referring to google’s question). I’ll use Baye’s Theorem to arrive at the same result as the given answer. Let Event A = having the disease. Let A’ = not having the disease. Let Event B = Positive result. Let Event B’ = Negative result. Given : P(A)=0.01, P(A’) = 0.99, P(B|A)=0.80, & P(B|A’)=0.20. Given that the result is positive we use Baye’s Theorem to calculate P(A|B) or the probability of having the disease given a positive result. Hence,

P(A|B) = [P(B|A)*P(A)]/[[P(B|A)*P(A)]+[P(B|A’)*P(A’)]]

= [0.80*0.01]/[[0.80*0.01]+[0.20*0.99]]

= 0.0388 or 3.88%

Note that the 1% was only the marginal probability of the population having the disease and the 80% was only the accuracy of the machine or having a positive result given a disease but there is still a 20% chance that the result will still come positive given that there is really no disease, thus, becoming a false positive. So the likelihood of someone having a disease given a positive result is not as simple as 80%( as what you have answered). We need to calculate the conditional probability, hence, P(A|B) = 0.388 or 3.88%

Thank you 🙂

I got the last part a typo – It should have been P(A|B)=0.0388 or 3.88%

an edit button should have been added.