[rerdavies]

An interesting parallel to that is the "What's the next number in this sequence?" sort of questions.

If four numbers are provided, one can calculate the coefficients of a a quartic polynomial, for x values of 0, 1, 2 and 3, and then solve for x=4. Which does indeed provide a defensible "next number". And by similar reasoning, there are an infinite number of answers to this question.

Even worse. You could in fact provide any number as an answer, because there is always a quintic polynomial that fits the four initial numbers AND your arbitrary fifth number.

So these questions are actually not about what the next number is, but trying to imagine what the person who set the question thought was a "cool" answer, for some curious definition of "cool", for some person who isn't smart enough to realize that the premise on which the question is based is flawed.