[krisoft]

> If this were a question on a grade school word problem test then just say 100

Let me repeat the question again: "If 5 machines can produce 5 parts in 5 minutes, how long will it take for 100 machines?" Do you think that by adding 95 more machines they will suddenly produce the same 5 parts 95 minutes slower?

What kind of machine have you encountered where buying more of them the ones you already had started working worse?

> then I would challenge you to come up with a version that's specified rigorously enough for any sufficiently picky interviewee.

This is nonsense. The question is under specified. You don't demonstrate that something is underspecified by formulating a different well specified question. You demonstrate it by showing that there are multiple different potentially correct answers, and one can't know which one is the right one without obtaining some information not present in the question.

Let me show you that demonstration. If the machines are for example FDM printers each printing on their own a benchy each, then the correct answer is 5 minutes. The additional printers will just sit idle because you can't divide-and-conquer the process of 3d printing an object.

If the machines are spray paint applying robots, and the parts to be painted are giant girders then it is very well possible that the additional 95 paint guns make the task of painting the 5 girders quasi-instantaneous. Because they would surround the part and be done with 1 squirt of paint from each paint gun. This classic video demonstrates the concept: https://www.youtube.com/shorts/vGWoV-8lteA

This is why the question is under specified. Because both 1ms and 5 minutes are possibly correct answers depending on what kind of machine is the "machine". And when that is the case the correct answer is neither 1ms nor 5 minutes, but "please, tell me more. There isn't enough information in the question to answer it."

Note: I'm struggling to imagine a possible machine where the correct answer is 100 minutes. But I'm sure you can tell what kind of machine you were thinking of.

[Drupon]

Not sure what you mean by this.

[krisoft]

I encourage you to ask a questions so I can figure out what do you not understand.

Let me also simplify my comment: “100 minutes” is not the correct answer to that question.