Java if statement not working

In my code I'm trying to make an if statement to have my final grades be equal to my theory and or practical if 1 or both of them are below 50%. If both are under 50%, the lowest one needs to equal the final grade. NOTE: I put a lot brackets in the code that i cannot get working if this helps.

My Code:

import java.text.DecimalFormat;
import java.util.Scanner;

public class GradeCalculator {
private double lab = 0;
private double quiz = 0; 
private double assignment = 0; 
private double midterm = 0; 
private double exam1 = 0; 
private double theory = 0;
private double practical = 0;
private double final_grade = 0;
private double grade_letter = 0;
private Scanner keys = new Scanner(System.in);


public GradeCalculator(){
    System.out.println("Welcome to the CST8215 Final Mark Calculator\n");
}

public void getMarks() {
    System.out.print("Enter your Lab mark out of 10: ");
    lab = keys.nextDouble();

    System.out.print("Enter your Quiz/Test mark out of 10: ");
    quiz = keys.nextDouble();

    System.out.print("Enter your Assignment mark out of 20: ");
    assignment = keys.nextDouble();

    System.out.print("Enter your Midterm mark out of 20: ");
    midterm = keys.nextDouble();

    System.out.print("Enter your Final Exam mark out of 40: ");
    exam1 = keys.nextDouble();

}

public void calculateGrades() {
    theory =((quiz + midterm + exam1)/70)*100;
    practical =((assignment+lab)/30)*100;
    final_grade =((lab+quiz+assignment+midterm+exam1)/100)*100; 
    grade_letter =((lab+quiz+assignment+midterm+exam1)/100)*100;    

}

public void displayGrades() {
    DecimalFormat number = new DecimalFormat ("##.##");
    System.out.print("\nTheory grade: "+number.format(theory)+ "%");
    System.out.print("\nPractical grade: "+number.format(practical)+ "%");
    System.out.print("\nFinal grade: "+number.format(final_grade)+ "%");
    System.out.print("\nGrade Letter: ");


    (((((   if(theory < 50 ) {
            final_grade = theory;
           }
            else if (practical < 50)
            final_grade = practical;   ))))))


        if(grade_letter < 60) {
        System.out.print("F");

     } else if (grade_letter >= 60 && grade_letter < 62.99) {
         System.out.print("C-");

     } else if (grade_letter >= 63 && grade_letter < 66.99) {
         System.out.print("C");

     } else if (grade_letter >= 67 && grade_letter < 69.99) {
         System.out.print("C+");

     } else if (grade_letter >= 70 && grade_letter < 72.99) {
         System.out.print("B-");

     } else if (grade_letter >= 73 && grade_letter < 76.99) {
         System.out.print("B");

     } else if (grade_letter >= 77 && grade_letter < 79.99) {
         System.out.print("B+");

     } else if (grade_letter >= 80 && grade_letter < 84.99) {
         System.out.print("A-");

     } else if (grade_letter >= 85 && grade_letter < 89.99) {
         System.out.print("A");

     } else if (grade_letter >= 90 && grade_letter < 100) {
         System.out.print("A+");

    } 

}
}