How many grams of potium permanganate should be added? (I have the answer but I don't know how to get it)?

How many grams of potium permanganate (KMnO4) should be added to a 100 volumetric flask to prepare a 0.02505 M solution when the flask is diluted to the mark with water? The answer is 0.3939 grams. How do I get to this answer?