How do you know that it's OK to make assumptions like that? How do you know that a quick estimate is acceptable? Is this level of accuracy acceptable? How do you know?

I can't get the point of this.

The interviewer wants to know how many golfballs fit on the bus, but he doesn't even know what kind of bus? He hasn't decided what things are on the bus, and how many of them, and whether there is a driver, and what kind of track he's going to be driving the bus on? And he probably doesn't know why the golfballs are being put on a bus at all? And he doesn't care as to what kind of accuracy this estimate is being made, as long as it just seems I've gone through some kind of motions?

This is ludicrous. It doesn't resemble any real-world task.