A mathematical puzzle whose solution is heavily based on the arithmetic properties of the integers. General number theory questions are off-topic but can be asked on Mathematics Stack Exchange.
Number theory is a branch of pure mathematics devoted primarily to the study of the integers. Sometimes number theory is called "The Queen of Mathematics" because of its foundational place in the discipline. Number theorists study prime numbers as well as the properties of objects made out of integers or defined as generalizations of the integers.
Note that general number theory questions are off-topic, but can be asked on our sister site Mathematics Stack Exchange.