What volume of 0.40 m na,s,o, would be required to react with the l, liberated by adcing excess of kl to 50 ml of 0.20 m cuso, solution ? 5 moil at@eduincle.com

mine's answers is coming 12.5 ml but it is given 25 ml in answer .The process which I did was first I estimated out the amount of iodine liberated by reacting copper with KI and then using the amount of Iodine liberated I calc out the volume of Na2S2O3 but i m not getting the answer...