I didn't add anything to the arrays, but my quiz still works fine. Is my answer okay?

// Setup the questions. Note that these all have int answers.
var questions = [
  ['How many states are in the United States?', 50],
  ['How many continents are there?', 7],
  ['How many legs does an insect have?', 6]
];
var correctAnswers = 0; // How many correct answers.
// The next three variables aren't strictly necessary, they're more for readability. 
var question; // Current question.
var answer; // Current correct answer.
var response; // Current user response.
var correct = []; // Array for correct questions.
var wrong = []; // Array for incorrect questions.

// This is the same as what you're using.
function print(message) {
  var outputDiv = document.getElementById('output');
  outputDiv.innerHTML = message;
}

// This builds out a block of HTML using the given array.
// At the end, he passes in the correct and wrong arrays created above.
function buildList(arr) {
 var listHTML = '<ol>'; // Create an ordered list for the type of answers (correct/wrong).
  // For each answer in the current array passed, build a list item.
  for (var i = 0; i < arr.length; i += 1) {
    listHTML += '<li>' + arr[i] + '</li>';
  }
  listHTML += '</ol>';
  return listHTML; // Return the HTML block.
}

for (var i = 0; i < questions.length; i += 1) {
  // These are the variables created for readability. The way you did it is okay too.
  question = questions[i][0];
  answer = questions[i][1];
  response = prompt(question);

  // You *do not* need this part.
  // Your toLowerCase() is the appropriate way to parse *your* questions.
  response = parseInt(response); // They have int-based questions.
  if (response === answer) {
    correctAnswers += 1; // ++ is just as good as doing it this way.
    correct.push(question); // They're using push(), because they're storing it in an array.
  } else {
    wrong.push(question); // push() here as well, for the same reason.
  }
}

html = "You got " + correctAnswers + " question(s) right." // Pretty much the same.

// Here is where their HTML gets built. This calls the buildList function created above.
html += '<h2>You got these questions correct:</h2>';
html += buildList(correct); // Passes in the created correct answers array.
html += '<h2>You got these questions wrong:</h2>';
html += buildList(wrong); // Passes in the created wrong answers array.
print(html);