A tablet with mass 0.940 g was dissolved in dilute sulfuric acid made up to 250 cm³ with water. 25.0 cm³ of this solution was titrated with 0.00133 mol dm⁻³ KMnO₄ requiring 32.5 cm³ of the KMnO₄. Calculate the percentage by mass of Fe²⁺ in the tablet.