Suppose you have four cards such that each card has a number on one side and a colour on the other side. Suppose that the four cards lay on the table in such a way to show you the faces 5, 8, blue, green, respectively. You are asked to decide the truth of the proposition P: if a card has an even number on one side, then the other side is blue, by turning exactly two cards. Which cards should you turn ? Justify carefully.