title: Converted Int returns NaN
body: <p>Here is my code </p>
  
  <pre><code>var fifty = prompt("Enter amount ");
  var twenty;
  alert(twenty=parseInt(fifty/50) + " x " +   "50 dollar bill");
  alert(parseInt(twenty/20) + " x " + "20 dollar bill");
  </code></pre>
  
  <p>What I'm trying to count is dollar bills.
  For example when I enter "120" it should return 2x 50 bills and 1x 20 dollar bill, I understand that returned value is String type so I'm converting them number, but on 20 dollar bills it returns "Nan x 20 dollar bils" I'm  having trouble understanding why</p>