The no. Of milliliters of 0.500 Na2S2O3 solution required to react completely with 2.0×10-3mol of I3-
I3^- + 2S2O3^2- ==> S4O6^2- + 3I^-
Is that 0.5 M Na2S2O3? You don't say M, N, %. I assume M.
You have mols I3^-.
Look at the equation. The coefficients tell you 1 mol I3^- requires 2 mols Na2S2O3
Then M Na2S2O3 = mols/L. You know M and mols, solve for L and convert to mL.