An iron tablet of mass $0.500\ \text{g}$ is dissolved in acid, and the iron is all in the form $\text{Fe}^{2+}$. It needs $17.4\ \text{mL}$ of $0.0210\ \text{mol/L}$ permanganate. The molar mass of iron is $55.85\ \text{g/mol}$. What is the percentage of iron by mass?