The question is a weird way of asking "do you know the definition of expected value?" Expected value is calculated by taking each possible outcome, multiplying that outcome's value by its probability of occurring, and summing the results. So to calculate for a six-sided die, where each outcome has probability 1/6, you simply multiple each possible outcome by 1/6 and sum those results:
Of course no actual roll of the die will ever come up 3.5, but as you roll the die more and more times, the average of all results seen so far will begin converging on 3.5 (and given infinite rolls, that's where it would end up).
I'll be honest, that's a really dumb question for testing programming skill.