> So yes, it mostly gets done for (components of) planes, spacecraft, medical devices, etc.
I have to disagree here. All of these you mentioned have regularly bugs. Multiple spacecraft got lost because of these. For planes there's not so distant Boeing 737 MAX fiasco (admittedly this was bad software behavior caused by sensor failure). And medical devices, the news about their bugs semi-regularly pop up. So while the software for these might do a bit better than the rest, they certainly are not anywhere close to being bug free.
And same goes for specifications the software is based on. Those aren't bug-free either. And writing software based on flawed specification will inevitably result in flawed software.
That's not to say we should give up on trying to write bug free software. But we currently don't know how to do so.
Unlike the car question, where you can assume the car is at home and so the most probable answer is to drive, with the machines it gets complicated. Since the question doesn't specify if each machine makes one part or if they depend on each other (which is pretty common for parts production). If they are in series and the time to first part is different than time to produce 5 parts, the answer for 100 machines would be the time to produce the first part. Where if each machine is independent and takes 5 minutes to produce single part, the time would be 5 minutes.
Kinda related question, but is code really just a math? Is it possible to express things like user input, timings, inteerupts, error handling, etc. as math?
I have to disagree here. All of these you mentioned have regularly bugs. Multiple spacecraft got lost because of these. For planes there's not so distant Boeing 737 MAX fiasco (admittedly this was bad software behavior caused by sensor failure). And medical devices, the news about their bugs semi-regularly pop up. So while the software for these might do a bit better than the rest, they certainly are not anywhere close to being bug free.
And same goes for specifications the software is based on. Those aren't bug-free either. And writing software based on flawed specification will inevitably result in flawed software.
That's not to say we should give up on trying to write bug free software. But we currently don't know how to do so.