[skeaker]

If your first paragraph was actually in the question then I would agree, but in the actual question, the reason there are exactly 100 people in line is that the question says there are 100 people in line. Phrasing certain parameters in your question as though they are absolutes and then expecting the person answering the question to change those parameters is like the worst kind of trick question. It's like when a toddler asks you if a ninja or a pirate would win in a fight, then says the pirate would win because he secretly has super strength. It's cute when a toddler does it because they innocently don't understand why their question wouldn't logically convey that hidden parameter to the listener, but inexcusable for a professor.

[SilverBirch]

I think you're just reading an ambiguous sentence in a way that obviously wouldn't make sense in context. The question is in a very familiar form: describe a scenario with some assumptions, describe a peturbation, ask what impact the peturbation has. If the context was some annoying internet quiz, the answer might have been a gotcha that the number of people in the queue stay the same.... but this is an econ question it's going to involve how people respond to economic incentives.

[hgsgm]

This is like those LeetCode questions in job interviews, or the interviewer wants a complicated O(n) solution when the problem as stated would be better solved with a simple O(n^2) solution because the maximum problem size is tiny.

It's a mind-reading game of which assumptions the professor/interviewer wants to keep solid which ones are open to change.