This is an easy one - a more interesting one is in the works!
A party is being held at a local mansion. The host is very rich and his success is because of one thing - his famous recipe for spaghetti.
The only guests that may attend are people who correctly reply to the guard at the door.
You are trying to gatecrash the party in order to steal the recipe. You sneak by and listen to the passwords, hoping to guess the pattern.
The first guest arrives. The guard says "3". The guest replies "5" and is admitted.
The second guest arrives. The guard says "1". The guest replies "2" and is admitted.
The third guest arrives. The guard says "12". The guest replies "9" and is thrown into a tank of red herrings to be devoured.
The fourth guest arrives. The guard says "21". The guest replies "7" and is admitted.
You walk up and get a "20" from the guard. What should you say?