I can't seem to concatenate two strings together in the append() method

$('resultsBlue').append('<p><b>Verschil: </b>' + (randomNumber1 - randomNumber) / 1000 + ' Seconden</p>');

#resultsBlue {
  background-color: #6699ff;
  border-radius: 7px;
}

.resultDiv {
  height: 200px;
  width: 300px;
  display: inline-block;
}

<script src="http://ift.tt/1oMJErh"></script>
<div id='resultsBlue' class='Div'>
  <h1>Blauw</h1>
</div>

That's a snippet of all of the code that has to do with this issue

As you can see in the snippet, I'm trying to append multiple data types but it just doesn't seem to work. How do I get it to do what I want? Thank you in advance