This definition is actually to let P/Q define all rational numbers without repetition.

First of all, according to the definition, P and Q must be prime numbers, so P=20 and Q = 5； cannot be taken. To get the rational number 4, take P=4 and Q= 1. Otherwise, rational number 4 can have countless definitions, such as 8/2, 12/3, 16/4. ...

This definition allows each rational number to have only one definition method. For example, 1 can only be defined as11,and 2 can only be defined as 2/ 1. You can give another example if you don't believe me.