A substance · 'A' undergoes disproportionation reaction to form $A^{3+}$ and $A^{2-}$ obeying first order kinetics. The rate · constant is obtained by measuring volume of an oxidizing agent capable of oxidizing A, $A^{3+}$ and $A^{2-}$ $A^{2-}$ to $A^{2+}, A^{4+}, A^{1+}$respectively. From the following data, calculate moles of $A^{2-}$ 6 minutes after start of the reaction if initially 10 moles of A was taken.