Make everything clear, step by step. I will upvote, Thank you

You have a stock solution of Catalase Enzyme in buffer that has a concentration of 400,0 U/mL (Units per Liter). You are tasked with making a 1 ml solution of 20,0 U/ml Catalase. What volume of Buffer must be added to what volume of 400.0 U/ml catalase to make ime of 20.0 U/mL catalase