122. Solving a chemistry problem

The decomposition of hydrogen peroxide in aqueous solution is a first order reaction. The amount of peroxide in the reacting mixture was determined by titrating samples of the same volume with a KMnO4 solution. At the initial moment of the reaction, 2208 ml of KMnO4 was used for titration after 10 minutes - 13.8 ml, after 20 minutes - 8.25 ml. calculate the average value K of the rate of this reaction.