The Question: Why must a calculated answear generally be rounded?

My Answear: A calulated answear cannot be more precise than the least precise measurement from which it was calculated.

I think your answer is ok but I think you are leaving out information that should be included. I would add a sentence something like this---put it in your own words. "Calculators and manual calculations may provide many more digits than are significant; thus, rounding is necessary to prevent a false impression." I would leave your first sentence as is.