A man wanted to enter an exclusive club but didn't know the secret password. He waited by the door and listened. A club member knocked on the door and the doorman said, "Twelve." The member replied, "Six" and was let in. A second member came to the door and the doorman said, "Six." The member replied, "Three" and was let in. The man thought he had heard enough and walked up to the door. The doorman said, "Ten," and the man replied, "Five," but he wasn't let in. What should he have said?

Explain.