In the Middle Ages, a spy from the court of King Arthur had the mission of entering a castle to investigate the plans of the enemies but upon arrival he discovered that the doors of the castle were closed and that an armed watchman controlled everyone who He was approaching so he hid among some bushes to watch.
A little while later a soldier arrived at the gates of the castle and the watchman of the tower said to him: eighteen! to which the soldier replied; nine! The doors opened and the soldier passed. Shortly after another soldier arrived and the lookout told him fourteen! and the soldier replied: seven! and they opened the doors to let the soldier in. Later another soldier arrived and the lookout said: eight! and the soldier replied: four! and they opened the doors again.
The spy of Arturo who had been watching thought that he had it very easy to enter so he approached the door and at that moment the watchman said: six !, to which the spy answered: three !. The lookout immediately called the guard and they captured him.
What did the spy have to have said in order to enter the castle? Why?
The spy mistakenly thought that the key used by the soldiers to enter was to say the number given by the lookout divided by two. Actually, the key was the number of letters in the word pronounced by the lookout. In this case the key that the spy should have said to enter was "four" since it is the number of letters of the word "six" pronounced by the lookout.