One litre of $0.15\, M\, Na _{2} SO _{3}$ aqueous solution is mixed with $500\, mL$ of $0.2\, M\, K _{2} Cr _{2} O _{7}$
aqueous solution in acid medium. What is the number of moles of $K _{2} Cr _{2} O _{7}$ remaining in the solution after the reaction?