C++ GPA program help (in Off-topic)

#include 
#include 
using namespace std;


void ProgrammerNote(void);
// displays the message "Programmed by ."

char ReadLetterGrade(string prompt);
// displays the prompt and reads a letter grade from the keyboard

double ReadCreditHours(string prompt);
// displays the prompt and reads the course credit hours from the 
// keyboard

string ReadName(string prompt);
// dipslays the prompt and reads the first and last names of the user
// from the keyboard. The first and last names are contatenated and the
// full name is returned.

double ComputeQualityPoints(double& letterGradeSum, double& creditHours);
// computes and returns the quality points for a given course.
// Quality points = point value of letter grade * credit hours

double ComputeGPA(double qualityPoints, double creditHours);
// computes and returns the gpa. gpa = quality points / credit hours

double pointsVal(char letterGrade, double& letterGradeSum);
// Gets the point value of the letter grade

void main(){
	char letterGrade, choice;
	double letterGradeSum, letterGradeNum, creditHours, qualityPoints, GPA = 0;
	string name;

	ProgrammerNote();
	do{
		name = ReadName("Enter student name [first last]: ");
		
		while (GPA == 0){
			letterGrade = ReadLetterGrade("Enter grade: [A-D, F]: ");
			
			if (letterGrade == 'A' || letterGrade == 'B' ||
				letterGrade == 'C' || letterGrade == 'D' || letterGrade == 'F'){

				letterGradeNum = pointsVal(letterGrade, letterGradeSum);

			
				creditHours = ReadCreditHours("Enter credit hours: [0.0]: ");
				creditHours += creditHours;	
			}

		
			else if (letterGrade != 'A' || letterGrade != 'B' ||
				letterGrade != 'C' || letterGrade != 'D' || letterGrade != 'F'){

				cout.setf(ios::fixed);
				cout.precision(5);
				
				qualityPoints = ComputeQualityPoints(letterGradeNum, creditHours);
				GPA = ComputeGPA(qualityPoints, creditHours);
				cout > choice;
	}while(choice == 'Y' || choice == 'y');
}

void ProgrammerNote(void){
	cout > letterGrade;

	if (letterGrade > creditHours;
	
	return (creditHours);
}

string ReadName(string prompt){
	string word, word2;

	cout > word >> word2;

	return (word + " " + word2);
}

// double ComputeQualityPoints(char letterGrade, double creditHours)
double ComputeQualityPoints(double& letterGradeSum, double& creditHours){
	double qualityPoint;

	qualityPoint = letterGradeSum * creditHours;
	return (qualityPoint);
}

double ComputeGPA(double qualityPoints, double creditHours){
	double GPA;

	GPA = qualityPoints / creditHours;

	return (GPA);
}