Change random operator or variable

I have an assignment in my programming class. It goes like this, I have to make a program which takes the input of the user how many exercises he wants to do. Then he solves simple calculations with random numbers from 1-10 and with random operators. In the end, it should write how many correct and incorrect ones did he get. It also should write the elapsed time of the task. I did some work, but when I assign a random value to an operation

int operation = (int)(Math.random()*3)+1;

or to a number a and b

int a = (int)(Math.random()*10); int b = (int)(Math.random()*10);

I always get the same number and operator when I choose for the second or third time my task (because I use a loop). Is there a way to change the same initialized variable or operator during a program. For example that int a=(int)(Math.Random()*10) is initialized in the beginning as for example as 3, and later when the program loops again to initialize it as a different number, for example 6. Are there any others solutions for my problem? Here is my whole code, for now:

    import java.util.*;
    import javax.swing.JOptionPane;
    public class RandomChar {


public static void main(String[] args) {

    char op= ' ';
    int operation = (int)(Math.random()*3)+1;

    int a = (int)(Math.random()*10);
    int b = (int)(Math.random()*10);
    String s;
    int correct = 0, incorrect=0;

    s = JOptionPane.showInputDialog("How many exercises do you want?");
    int num = Integer.parseInt(s);

    long tStart = System.currentTimeMillis();

    while(num>0){

        if(operation==1)
        op='+';
    else if(operation==2)
        op='-';
    else if(operation==3)
        op='*';

    String str1 = JOptionPane.showInputDialog(a+" "+op+" "+b+" = ");
    int num1 = Integer.parseInt(str1);

    if(op=='+'){
    if(a+b==num1)
        correct++;
    else
        incorrect++;
    }else if(op=='-'){
        if(a-b==num1)
            correct++;
        else
            incorrect++;
    }else if(op=='*'){
        if(a*b==num1)
            correct++;
                    else
            incorrect++;
    }
    num--;
    }

    long tEnd = System.currentTimeMillis();
    long tOverral = tEnd - tStart;
    double elapsedSeconds = tOverral / 1000.0;
    System.out.println("Correct: "+correct);
    System.out.println("Incorrect: "+incorrect);
    System.out.println("Elapsed seconds: "+ elapsedSeconds);


}

}