Q: Number of fire hydrants in NY within 1 year.
Hi, there,
Very interesting question. I got one of those in my final round so I tend to like those sorts of questions. First I will let you with a general 6-step approach for this type of question and then I will let some food for thought.
Approach:
1. Validate the understanding and scope of the question with the interviewer. As a rule of thumb, all such problems are open to different interpretations so you should clarify it before jumping to any calculations. For instance, you could clarify if you are being asked the number of hydrants sold, or installed, hydrants of which type, if repairs should count or not, in which part of NY, etc. You are expected to come up with clarifying questions.
2. Lay down and structure the ways you can come up with a solution (without numbers). You might come up with 2 or even 3 different ways to approach the problem sometimes.
3. Communicate and validate the proposed approach you want to take with the interviewer.
4. Do the calculations and state premises out loud, always checking if it sounds fair with the interviewer, as Charlotte pointed out. This part should be a back-and-forth interaction with the interviewer, since he/she will probably engage with small comments, validations, challenges and/or suggestions.
5. Once you get to the answer, re-state the question in one sentence and present the answer clearly.
6. Perform an out-loud judgement of your answer, show the interviewer you are looking for validating whether your answer makes sense against other information you already know (GDP, population, etc.)
Food for thought: although there is no right or wrong approach or answer, I will just state some that come to mind. Assuming we are talking about new fire hydrants sold in 2023 in the main neighborhoods of NYC. I will use FH for fire hydrants.
1. # FH per year = # existing FH / lifecycle (in years) -> for the # existing FH , you can go by taking the size of NYC in km2 and use a premise on number of FH per km2. You can split the regions if you think the number of FH per km2 is significantly different in each one.
2. Do the same but use a premise in terms population size of NYE and number of hydrants per capita.
3. If you know the revenues of FH company and a guess on its market share you can do: # FH / year = ($ Revenues / mkt share) / # avg. FH price.
I hope it helps! Best of luck and please reach out if you have further questions.
Cheers
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