Brain Teaser Of The Day: The Club Password

A man wants to enter a club, but there is a password he does not know. He watches one guest approach. The doorman says “12” and the guest says “6”. Next, guest comes and the doorman says “6.” The guest says “3”. Seeing the pattern, the man approaches. The doorman says “10” so the man says “5.” He is denied entry. What should he have said?

Brain Teaser Of The Day: The Club Password