What is The Password Trick?

A young man was standing outside a bar thinking to enter into bar. But there was a one problem. There was a guard standing in door asking password. Only those producing correct password allowed to enter. The young man stood there & started observing carefully. A man came to enter into bar. The guard said - twelve, the man answered six. Man allowed to enter. A lady came, guard said - six; she replied - three.The guard allowed her to enter.


Now thinking that he has cracked the trick of password generation the young man went to the guard. The guard said - ten, immediately the young man replied - five. But the guard denied his entry to the bar. Can you tell the reason? 

Decode the password generation process!
What's the password?

This was the trick! 

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