[Solved] Javascript add value if checkbox is checked

var chap7 = document.getElementById('chap7'),
    post = document.getElementById('post'),
    printed = document.getElementById('printed');

printed.addEventListener('change', function() {
  var quantity = parseInt(chap7.value, 10);
  
  post.value = printed.checked ? quantity * 3.5 : quantity;
}, false);

chap7.addEventListener('change', function() {
  post.value = parseInt(this.value, 10);
});

<p>
    <label for="hours">Learn JavaScript the Hardy Way’ - Complete e-book (Printed)</label>
    <input type="number" min="0" id="chap7" name="hours" value="0">
</p>

<input type="checkbox" id="printed"/>Please tick if you would like a printed version<br />

<p class="post">Postage : <strong>£<output id="post">0</output></strong><p>

var chap7 = document.getElementById('chap7'),
    post = document.getElementById('post');

chap7.addEventListener('change', function(e) {
  var quantity = parseInt(chap7.value, 10);
  
  post.value = quantity * 3.5;
}, false);

<p>
    <label for="hours">Learn JavaScript the Hardy Way’ - Complete e-book (Printed)</label>
    <input type="number" min="0" id="chap7" name="hours" value="0">
</p>
<p class="post">Postage : <strong>£<output id="post">0</output></strong><p>