Can I just say "I assume it's about 2000." and be done with it? I mean, that's a good enough estimate when we're not actually going to be putting golf balls on buses and are just chatting about it.
The purpose of the question is[2] to gauge the interviewee's ability to solve problems.
With those constraints your approach would appear to be sub-optimal. Sometimes in life we have[3] to jump through hoops to get what we want.
[1] Primary purpose; there are other things, such as deciding whether the company is a terrible place or full of idiots or etc.
[2] Should be; I accept that maybe many people have heard about these questions and think they're the right thing to ask but have no idea of the purpose.
[3] Or we decide that hoop jumping is not for us and we reject all hoop jumping and politely decline the job.
It's a matter of putting on a show for the interviewer. But we don't know what kind of show. A show where we make assumptions about things? or ask questions? A show where we try to get as 'real-world' an answer as possible? or one where we assume things to make our lives easier? Why not just assume it's a toy bus and nothing can fit in it? Is that a clever and funny time-saving technique, or am I mocking the interviewer?
But yeah, the candidate has to do what he has to do if he needs a job. I think that's what you're saying anyway.