Hi testtakerstrategy,
It might help to think about what exactly you are "averaging" and why. Here, we're averaging 2 things: "100 units of currency" and "1% of one's annual income", so you're probably thinking about either dividing by 2 or dividing by 101. Dividing by 101 wouldn't make much sense, since we're not asked to find the average of all of all 101 individual items (and both references are clearly in terms of the Country R's currency). The only other option would be to divide by 2, which would make sense, since we're trying to determine the calculation for the amount of income tax (and the prompt is clearly discussing the formula for that).
GMAT assassins aren't born, they're made,
Rich