public class HelloWorld{

    public static void main(String[] args) {

    int one = 10 ;

        int two = 20 ;

        int three = 0 ;

        three = one + two;

        System.out.println("three = one + two ==> " + three);

        three += one;

        System.out.print("three += one ==> " + three);

        three -= one;

        System.out.print("three -= one ==> " + three);

        three *= one;

        System.out.print("three *= one ==> " + three);

        three /= one;

        System.out.print("three /= one ==> " + three);

        three %= one;

        System.out.print("three %= one ==> " + three);

仔细看要求。他要的结果有好几个，而你只赋了一次值,也就是：three = one + two;

System.out.println后面括号中“”里面的东西都会以原模原样出现在结果中，真正输出的值是+后面的变量的值，所以你的代码应该是全都输出30吧