The concentration of Fe²⁺ ions in an acid solution can be determined by a redox titration with either KMnO₄ or K₂Cr₂O₇. The reduction products of these reactions are Mn²⁺ and Cr³⁺, and in each case the iron is oxidized to Fe³⁺. In one titration of an acidified Fe²⁺ solution, 25.20 mL of 0.0210 m K₂Cr₂O₇(aq) was required for complete reaction. If the titration had been carried out with 0.0420 m KMnO₄(aq), what volume of the permanganate solution would have been required for complete reaction?