There is a monastary in which the ihabitants, an order of perfectly logical monks, have taken vows of silence, and have sworn not to communicate with each other in any other way. There are also no mirrors or reflective surfaces in the monastary. One day a demon comes to the monastary and marks at least monks in the night. A monk marked by the demon will have 666 shown on his forehead. The next morning the demon says says "at least one of you is marked by me" and leaves. The monks, being devout monks, will kill themselves during the night if they know they are marked.
On the first night, none killed themselves
On the second night, none killed themselves
On the third night, none killed themselves
On the fourth night, none killed themselves
On the fifth night, a number of monks killed themselves
how many monks were marked and how do you know?
(yes it is solveable, no theres no 'trick' its pure logic, nothing to do with it being a monastary)