Grading Program - Calculate Final Grade By Using Weighted Scoring System

Many classes calculate a final grade by using a weighted scoring system. For example, "Assignments" might be worth (weighted at) 40% of your final grade. To calculate the portion of the final grade contributed by the Assignments category, the teacher takes the average of all assignments in the category and multiplies it by the weight (40%). So if a student averaged 90% on all assignments, the teacher would take 40% of 90%, or 36 as the weighted average score for the assignment portion of the the final score. The teacher then calculates the weighted average score for each of the other categories (Quizzes, Midterm Exam, Final Exam, etc.), adds them all together to come up with the final score, and assigns a letter grade based on that score.

Write a Java program that allows the teacher to calculate the grade for a student.

Specification :

Prompt the user to enter the Student ID number, first name, last name, and the average score for each of the following categories:

Assignments (worth 50%)
Quizzes (worth 20%)
Midterm Exam (worth 10%)
Final Exam (worth 20%)

Calculate the weighted average score for each category and the total score for the student. Next assign a letter grade as follows:

90 - 100 = A
80 - 89 = B
70 - 79 = C
60 - 69 = D
59 and below = E

Print the results in the following format:

Name: [Last Name], [First Name]
Student ID: [Student ID]
Assignments: [weighted average]
Quizzes: [weighted average]
Midterm Exam: [weighted average]
Final Exam: [weighted average]
Total: [total score]
Final Grade: [letter grade]

Prompt the user to either enter another student or to quit. Continue the process until the user chooses to quit.

I almost done everything but I cant calculate a total score for the student or Assign a letter grade based on the score!!!!
 
package teacher_grade;
import java.util.Scanner;
public class teachergradeing {
public static void main (String[] args) {
//Scanner objective

[Code] ....