title: jQuery - Change values when checkboxes are checked
body: <p>i have three checkboxes in HTML</p>
  
  <pre><code>&lt;input type="checkbox" id="a1"/&gt;
  &lt;input type="checkbox" id="a2" /&gt;
  &lt;input type="checkbox" id="a3" /&gt;
  &lt;span class="price"&gt;&lt;/span&gt;
  </code></pre>
  
  <p>I want to do this: 
  If id A1 is checked, change text in span to priceUp.
  If ids A1 and A2 is checked, change text in span to priceDown.
  If ids A1,A2 and A3 is checked, change text in span to priceNormal.</p>