This sounds like an example of prototype theory which describes how we categorise concepts.
From Wiki: "Prototype theory is a mode of graded categorization in cognitive science, where some members of a category are more central than others. For example, when asked to give an example of the concept furniture, chair is more frequently cited than, say, stool"http://en.wikipedia.org/wiki/Prototype_theory
In essence the majority of people, within a particular culture/s, will come up with the same answer when asked to name an object within a category, as this object is the most representative (in terms of their experience) of that category. I imagine this is one of the tools people like Derran Brown (a TV magician/mentalist) use to perform their 'mind reading' tricks. I read that other common answers are peas and cabbage.
Another example is to ask people to quickly name a colour and a tool.....
Supposedly most people say red hammer. I found this website which tested the theory (not exactly great science but its interesting!) which found the majority did say red hammer. They also showed that the maths problem given before the question had no effect.http://scienceblogs.com/cognitivedaily/ ... t_were.php
Although people are very susceptible to priming (i.e an answer is highly influenced by what the person has been exposed to before the question is asked) I cant see how maths would influence such an unrelated question. It may just be another magicians misdirection tool to make it appear to be somehow connected to the maths and throw people how the 'trick' works.
I read a paper a while back (though I dont have a reference to hand, Ill try to find it when I have more time!) that performed brain imaging (fMRI if I recall correctly) while asking people to name an object within a certain category. They showed there was less brain activity when the people named the typical response (i.e carrot/cabbage etc) compared to when they had to think of another object within the same category. This suggests that the common answer is somehow programmed into the brain as the first response and therefore takes less effort to retrieve the word. The extra brain activity that occurred when asked to name a second object suggests there is extra processing required to find this word and may take fractionally longer to retrieve. I imagine this is why the trick works when you need to give an immediate response, as the word that requires less brain processing will be the first one you say.
Im not sure why most people identify one object as the most representative of a category but I imagine its a lot to do with life experience. I would guess most people would say dog or cat if asked to name an animal and these are the animals people generally have the longest exposure to. I agree with the previous posts that the answers would vary depending on culture.