How do i make average grade refer to grade which i created in If-Else Statement

public static boolean gradeCalculator() {
    double mark,student;
    String grade = "";
    double total = 0;

    System.out.println("Enter number of students :");
    student = scanner.nextDouble();

    for(int i=0; i<student; i++){

    System.out.println("Enter mark for student "+ (i+1) +" : ");
    mark = scanner.nextDouble();
    total = total + mark;
    double average = total/student;
    {
      if(mark >=0 && mark <=39){
        grade = "F" ; 

         }
      else if(mark >=40 && mark <=49){
        grade = "P";
         }
      else if(mark >=50 && mark <=59){
        grade = "D";
        }
      else if(mark >=60 && mark <=69){
        grade = "C";
         }
      else if(mark >=70 && mark <=79){
        grade = "B";
         }
      else if(mark >=80 && mark <100){             
        grade = "A";
        }  
      System.out.println( "Grade : " +grade);
    }
}
    System.out.println( "Average Mark : " +total/student);
    System.out.println( "Average Grade : " );
    System.exit(0);
    return false;
}

I wanted to calculate the grade for students, and the average at the end of the script. But i'm wondering how do i make the average mark refer to the grade which i created previously in the if-else statement.