Hi every one

I am currently working on a quiz for a project. The code works but I want to add something else to it that I can't work out how to do. I want the quiz to mark the code and if the user gets over 80%a button is displayed allowing the users to move onto the next section. Any ideas? here is the code:
Code:
// Function to create arrays.
function makeArray(len) {
    for (var i = 0; i < len; i++) this[i] = null;
this.length = len;
}

// creates objects for this question.
function makeQuestion(question, correctAnswer) {
var args = makeQuestion.arguments;
this.question = question;
this.correctAnswer = correctAnswer;
this.userAnswer = null;
this.isCorrect = isCorrect;
this.showForm = showForm;
this.userChoices = new makeArray(args.length - 2);
   for (var i = 0; i < args.length - 2; i++) { 
   this.userChoices[i] = args[i + 2];
   }
}

// determins if the questions answered correctly
function isCorrect() {
if (this.correctAnswer == this.userAnswer) return true;
else return false;
}

// Method to display contents of question object.
function showForm(n) { //argument n is the number of the question.
document.write((n + 1) + '. ' + this.question + '<BLOCKQUOTE><FORM>'); //n+1 = the number of the question in the array (has to add one because the array starts with 0 or the first question would be number 0, this question displays the questions text, blockquote off sets the answer making them indented.
    for (var i = 0; i < this.userChoices.length; i++) { //goes through the questions one by one.
        document.write('<INPUT TYPE = "RADIO" NAME = "q' + n + //create the radio buttons.
                       '" onClick = "quiz[' + n + '].userAnswer = ' + i + '">');
        document.write(this.userChoices[i] + '<BR>');
    }
document.write('</FORM></BLOCKQUOTE>');
}

// 	This corrects the quiz, dispays the correct answers and the score.
function correctQuiz() {
var correct = 0; // sets the correct page to a score of zero
correctPage = '<HTML><TITLE>Corrections</TITLE><BODY BGCOLOR = "#FFFFFF">'; // sets the attributes of the correction page
    for (var i = 0; i < quiz.length; i++) { //loop runs through questions
    if (quiz[i].isCorrect()) correct++; // the is correct starts at 0 every time is correct returns true (an answer is correct) it adds 1 to the number adding up correct answers.
    }
var score = Math.round((correct / quiz.length) * 100); // find out the % of correct answers by using math and the % equation
correctPage += 'Score: <STRONG>' + score + '</STRONG> %';
    if (correct < quiz.length) {  //if the correct number is less than the quiz length then not all the answers were correct if it is the same then all ansers were correct.
    correctPage += ifWrong; // if there were wrong answers
        for (var i = 0; i < quiz.length; i++) {  // starts a for loop through the answers
            if (!quiz[i].isCorrect()) { // if quiz doesn't = to iscorrect.
            correctPage += (i + 1) + '. ' + // this shows what number was wrong.
            quiz[i].userChoices[quiz[i].correctAnswer] + '<BR>'; // displays the correct answes for the questions that were wrong
            }
        }
    }
    else correctPage += ifAced;  // this is the correct page displayed if all the answers were right.
correctPage += '</BODY></HTML>'; // opens the window of correct page
correctwin = window.open ('', '', 'height=300,width=300,scrollbars=yes'); 
    if (correctwin.opener == null) correctwin.opener = window;
correctwin.location = 'javascript:opener.correctPage';
}

// Message to display if quiz is aced.
var ifAced = "<P>Wow you really were paying attention well done you ACED it.<P> please click here to carry on"; 
// Message to display if any are wrong.
var ifWrong = "<P>FAILED!!!!!! did you fall asleep whilst you were browsing my site??? Don't bother retaking it here are the answers!<P>";

// Create seven question objects...
quiz = new makeArray(8); // this is how many questions there is in the quiz 
quiz[0] = new makeQuestion("What is the correct name for the leg marking found in picture 1?", // 1st question  starts at 0 because thats the first number in an array
                            2,                             // correct answer           
                           "Sock",                // choice 0
                           "Stocking",                // choice 1
                           "Coronet band",                    // choice 2         
                           "half cannon");    // choice 3        

quiz[1] = new makeQuestion("What is the correct name for the leg marking found in picture 2?",         // 2nd question 
                            0,                             // correct answer
                           "Sock",    // choice 0
                           "Stocking",        // choice 1
 	        			   "Coronet band", //choice 2
                           "half cannon"); //choice 3

quiz[2] = new makeQuestion("What is the correct name for the leg marking found in picture 3?",         
                            1,                             // correct answer
                           "Sock",    // choice 0
                           "Stocking",        // choice 1
 	        			   "Coronet band", //choice 2
                           "half cannon");

quiz[3] = new makeQuestion("What is the correct name for the leg marking found in picture 4?",
                             3,                             // correct answer
                           "Sock",    // choice 0
                           "Stocking",        // choice 1
 	        			   "Coronet band", //choice 2
                           "half cannon");

quiz[4] = new makeQuestion("What is the correct name for the face marking found in picture 5?", 
                            2, 
                           "Bald",
                           "Star", 
                           "Snip",
                           "Blaze");

quiz[5] = new makeQuestion("what is the correct name for the face marking found in picture 6?",
                              0, 
                           "Bald",
                           "Star", 
                           "Snip",
                           "Blaze");

quiz[6] = new makeQuestion("what is the correct name for the face marking found in picture 7?",
                              3, 
                           "Bald",
                           "Star", 
                           "Snip",
                           "Blaze");
quiz[7] = new makeQuestion("what is the correct name for the face marking found in picture 8?",
                             1, 
                           "Bald",
                           "Star", 
                           "Snip",
                           "Blaze");