I think both answers are poor. The "weak" one answers the question, "How many lamps are there in a city street 100 blocks long?" Obviously that was not the ques more...

I think both answers are poor. The "weak" one answers the question, "How many lamps are there in a city street 100 blocks long?" Obviously that was not the question asked and is wrong, but how correct is the "strong" one? During the answer, the interviewee jumps from assuming 10 lamps per street to 10 lamps per city block (!). In reality, there are four streets to a city block, one on each side, thus creating a huge error! In addition, no account is made for those interior streets, the vast majority of the grid, that are shared by adjacent city blocks. So multiplying by 10,000 by 4 to solve the first error and then dividing by 2 to account for the second would give you the low estimate 20,000 lamps, although 2,000 or so needs to be added to this figure to obtain the true number. I my mind, the errors in the strong answer are as egregious as the over-simplification in the weak answer. ...less

Thanks for sharing. Recently, during an interview I was asked to estimate the number of Interstate highways in St. Louis City; My answer was straightforward, bu more...

Thanks for sharing. Recently, during an interview I was asked to estimate the number of Interstate highways in St. Louis City; My answer was straightforward, but I could just realise after that it was a poor response. ...less

there are some visible code comments on this page. ( see below)

Strong Answer

Let’s assume Manhattan more...

there are some visible code comments on this page. ( see below)

Strong Answer

Let’s assume Manhattan is 100 blocks long and 10 blocks wide, so there are 1000 square blocks in the city. If in each square block there are 5 lamps on each side of the street, 10 total, then the total number of street lamps would be 10 lamps for each square block times 1000 square blocks, or 10,000 street lamps.



Commentary

This is a very straightforward question and variations of this are common (how many cups of Starbucks coffee are consumed in a given day in the U.S.? How many red cars might you find in the city of Los Angeles?). The interviewer wants to see if you can quickly and easily identify the key assumptions or variables and do some fairly simple math on the fly.

In the weak answer, the candidate makes a few assumptions and explains his/her calculation, but has mistakenly narrowed the scope too much, forgetting that there are streets which intersect the 100 blocks. As a result, the interviewee is significantly underestimating the likely number of street lamps.


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