4 ms·

I'm not sure I buy this type of attack. How is this different from calculating yield of a bomb as Fermi did? It's back-of-envelope calculation that physicists d

I'm not sure I buy this type of attack. How is this different from calculating yield of a bomb as Fermi did? It's back-of-envelope calculation that physicists do all the time. Ping Pong in a Boeing is merely a setup with the least context. This is like criticizing a math teacher for asking elementary students to calculate how long it takes to fill a swimming pool when water is flowing in and out of the pool at the same time. Yeah, the setup sounds ridiculous, but you know what, that's the setup that a student understands, and we see similar scenarios in real world: a company earns and spends at the same time, a queuing system has incoming traffic and completed tasks, a factory has incoming jobs and outgoing completed products... So, what's wrong with the question again? Frankly, the fictional Feynman in the article sounds confrontational and arrogant. Take the manhole question for an example, all the counter questions are either nit picking, or rejecting the value of industrial design. Of course there is a reason for choosing a round shape for manhole cover. Of course there is a trade-off among different shapes. Please note, I'm not saying the manhole question is good for interview, but it does not mean that we need to be mean and resort to cheap attacks to make a point.

rileymat2 5y ago
There is an interesting point in there about reverse engineering stuff, like you sometimes see in evolutionary biology and such.
derekp7 5y ago
This is fine if the question relates to something that the candidate is familiar with. But I have personally never set foot in a 747, much less be able to come up with any estimate of the dimensions in order to estimate volume. Then there is the formula to determine the packing volume of ping pong balls. Just like the question of how much water dumps out of the Amazon river per day. Yes, I can write down a formula with a bunch of variables, but there is no way I could justify my guess of what the values of those variables are without some research ahead of time. And in reality, who wants a developer that just starts banging out code without researching the problem domain space first? In Fermi's case he already knew the inputs such as air resistance, weight of the paper, and of course knew all the relevant math formulas because that was part of his education and job.
- giobox 5y ago
  I’ve found in interviews you can just declare your assumptions for the things you don’t know. Ultimately these sorts of job interview questions are designed to evoke a response that can let you demonstrate how you would go about solving/explaining the problem; actually solving it correctly is rarely required or expected. I’d just say having never seen a 747, for purposes of the calculation let’s assume they are 300m long with a 10m diameter etc etc and proceed to do a terrible job of calculating how many ping pong balls fit.
  - zinclozenge 5y ago
    You would think that ultimately the question is about the thought process. But that usually gets lost and it devolves into a question that gets asked with an expected answer.
- hintymad 5y ago
  I guess we can always clarify the context? Something like: I've never set foot in a 747, could you please describe what that is? Can I assume that the plane has no furniture, and the dimension is yada yada? You know, have a conversation. Actually, that's exactly what Feynman did in the excellent writeup: https://longnow.org/essays/richard-feynman-connection-machine/ https://longnow.org/essays/richard-feynman-connection-machin.... A quote: "For Richard, figuring out these problems was a kind of a game. He always started by asking very basic questions like, "What is the simplest example?" or "How can you tell if the answer is right?" He asked questions until he reduced the problem to some essential puzzle that he thought he would be able to solve."
- thinkharderdev 5y ago
  How well these questions correlate to job performance is an interesting question but the goal I think is not for you to get to the "right" answer (in the sense of figuring the optimal bin-packing approach on the fly). The goal is to see how you are able to take a broad problem and break it down by: 1. Identifying key assumptions or inputs. You may not know the dimensions of a 747 off-hand but ideally you can come up with a way of determining the number of ping-ping balls GIVEN the correct dimensions as input. That is you can come up with a model that you can plug the right numbers into once you look them up. 2. Clarifying the problem by asking questions and drilling in on requirements. 3. Create a model that you can communicate to others and allows you to make decisions just based on order-of-magnitude considerations. I.e. make decisions under uncertainty. All of those skills I think are relevant to software engineering and system design. For example, you may need to provision hardware long before you know the exact design of a system. Can you come up with a rough estimate of how much storage you will need? Or at least put reasonable upper bounds on it? How many qps would you expect a system to need to support and does that mean you have to use a distributed datastore vs a single RDBMS?
memco 5y ago
Why use the pool at all if you already have useful examples of inflow and outflow of money or another or the queue at the grocery store? Someone of age to understand flow rates of liquid would likely be able to understand money enough to be able to do the same calculations can’t get the same principles but with a functional analogy instead.
- nickff 5y ago
  There are an infinite number of variations on the problem; you could also ask about a reservoir with inflows and outflows, or a water tower, or a continuous distillation process. I think the money variant is actually one of the worst, as it evokes no imagery (in an era of largely digital transactions), but I'm not sure the exact question asked really matters. If someone fights a hypothetical, they're usually being adversarial, confrontational, or just avoiding answering the question, which is useful information in an interview.
  - FabHK 5y ago
    > I think the money variant is actually one of the worst, as it evokes no imagery Plus money is obviously discrete, while water is conceptually nice and continuous.
- hintymad 5y ago
  My guess is that such problems are for 3rd-graders, who do not necessarily have real-world experience to understand more realistic setup.
sjg007 5y ago
The cylinder being the strongest shape is the most interesting answer.

rileymat2 5y ago

There is an interesting point in there about reverse engineering stuff, like you sometimes see in evolutionary biology and such.

derekp7 5y ago

This is fine if the question relates to something that the candidate is familiar with. But I have personally never set foot in a 747, much less be able to come up with any estimate of the dimensions in order to estimate volume. Then there is the formula to determine the packing volume of ping pong balls. Just like the question of how much water dumps out of the Amazon river per day. Yes, I can write down a formula with a bunch of variables, but there is no way I could justify my guess of what the values of those variables are without some research ahead of time. And in reality, who wants a developer that just starts banging out code without researching the problem domain space first? In Fermi's case he already knew the inputs such as air resistance, weight of the paper, and of course knew all the relevant math formulas because that was part of his education and job.

giobox 5y ago
I’ve found in interviews you can just declare your assumptions for the things you don’t know. Ultimately these sorts of job interview questions are designed to evoke a response that can let you demonstrate how you would go about solving/explaining the problem; actually solving it correctly is rarely required or expected. I’d just say having never seen a 747, for purposes of the calculation let’s assume they are 300m long with a 10m diameter etc etc and proceed to do a terrible job of calculating how many ping pong balls fit.
- zinclozenge 5y ago
  You would think that ultimately the question is about the thought process. But that usually gets lost and it devolves into a question that gets asked with an expected answer.
hintymad 5y ago
I guess we can always clarify the context? Something like: I've never set foot in a 747, could you please describe what that is? Can I assume that the plane has no furniture, and the dimension is yada yada? You know, have a conversation. Actually, that's exactly what Feynman did in the excellent writeup: https://longnow.org/essays/richard-feynman-connection-machine/ https://longnow.org/essays/richard-feynman-connection-machin.... A quote: "For Richard, figuring out these problems was a kind of a game. He always started by asking very basic questions like, "What is the simplest example?" or "How can you tell if the answer is right?" He asked questions until he reduced the problem to some essential puzzle that he thought he would be able to solve."
thinkharderdev 5y ago
How well these questions correlate to job performance is an interesting question but the goal I think is not for you to get to the "right" answer (in the sense of figuring the optimal bin-packing approach on the fly). The goal is to see how you are able to take a broad problem and break it down by: 1. Identifying key assumptions or inputs. You may not know the dimensions of a 747 off-hand but ideally you can come up with a way of determining the number of ping-ping balls GIVEN the correct dimensions as input. That is you can come up with a model that you can plug the right numbers into once you look them up. 2. Clarifying the problem by asking questions and drilling in on requirements. 3. Create a model that you can communicate to others and allows you to make decisions just based on order-of-magnitude considerations. I.e. make decisions under uncertainty. All of those skills I think are relevant to software engineering and system design. For example, you may need to provision hardware long before you know the exact design of a system. Can you come up with a rough estimate of how much storage you will need? Or at least put reasonable upper bounds on it? How many qps would you expect a system to need to support and does that mean you have to use a distributed datastore vs a single RDBMS?

memco 5y ago

Why use the pool at all if you already have useful examples of inflow and outflow of money or another or the queue at the grocery store? Someone of age to understand flow rates of liquid would likely be able to understand money enough to be able to do the same calculations can’t get the same principles but with a functional analogy instead.

nickff 5y ago
There are an infinite number of variations on the problem; you could also ask about a reservoir with inflows and outflows, or a water tower, or a continuous distillation process. I think the money variant is actually one of the worst, as it evokes no imagery (in an era of largely digital transactions), but I'm not sure the exact question asked really matters. If someone fights a hypothetical, they're usually being adversarial, confrontational, or just avoiding answering the question, which is useful information in an interview.
- FabHK 5y ago
  > I think the money variant is actually one of the worst, as it evokes no imagery Plus money is obviously discrete, while water is conceptually nice and continuous.
hintymad 5y ago
My guess is that such problems are for 3rd-graders, who do not necessarily have real-world experience to understand more realistic setup.

sjg007 5y ago

The cylinder being the strongest shape is the most interesting answer.