My task works, but I don't understand something ?

A is defined in the same block of code as the main method (the Solution class block). This means the main method can access it without having to use its full qualified name (Solution.A). If you made another class (or block of code) inside this one, and had a static variable inside that, the main method would have to use the qualified name. Think of a block of code like a classroom full of students. If you said "John is a great person", and there was a John in the classroom, everyone would know who you were talking about. However if there was no John in the classroom people would ask "John who?", in which case you would have to use their full name so people knew which John you were talking about. Here is a quick example:

public class Solution {
    static int A;

    public static void main(String[] args){
        // not required here because A and main reside in
        // same block of code.
        A = 15;


       // in this case the qualified name is required
       // just typing A will result in an error because
       // the B is within a different code block
       test.B = 15;
    }

    static class test {
        static int B;
    }
}

In this bit of code you have 3 blocks of code: 1) lines 1-19, the Solution class block, which contains static int A, main() method, and test class 2) lines 4-14, the main method, nothing declared inside 3) lines 16-18, the test class, which contains static int B The way the rules work, as simple as possible, anything inside a block of code where an object is declared can use just the variable name. Anything outside the block of code would have to use the qualified name. I recommend looking into "Variable Scope" for more information. This is what this programming concept is called.