Physics: Current and Resistance?

Not quite: the cross sectional area of the tube is π(2 mm)² - π(1 mm)² = 3π mm². The cross-sectional area of the first is π(1 mm)² π mm². Since the resistivity and length remain the same, we have Ra/Rb = (1/Aa)/(1/Ab) = Ab/Aa = (3π)/π = 3.