0.120 g of pyrite, $\mathrm{FeS}{}_2$ (molar mass 120), is completely oxidised by acidified $\mathrm{KMnO}{}_4$ to $\mathrm{Fe}{}^{3+}$ and $\mathrm{SO}{}_4{}^{2-}$. The volume of 0.0200 M $\mathrm{KMnO}{}_4$ needed is:

In $\mathrm{FeS}{}_2$, Fe is +2 and each S is -1. Fe: +2 to +3 gives 1 electron; each S: -1 to +6 gives 7 electrons, so 14 for two S. n-factor = 15. meq = $1.00\times 15=15$. Normality of permanganate = 0.100 N, so V = 15/0.100 = 150 mL.