Bertrand has 12 cards: on the front of each card is a digit, and on the back of each card is either a triangle, square, pentagon or circle. Bertrand looks through the pile of cards and says to his friend Georg, "For each card in the pile, if the number on the front of the card is even, then the shape on the back is a circle". Bertrand then lays the cards on a table as below. Georg wants to determine whether Bertrand was telling the truth by examining the hidden side of some of the cards. What is the fewest number of cards that Georg must examine to determine whether Bertrand was telling the truth?
