title: Calculate with .val() of input jQuery
body: <p>i'm working on a project, and i have to make some calculations on an existing value of an input field. let's say the input value is 400 or whatever.</p>
  
  <p>underneath i've got a select box <strong>YES</strong> means add <strong>425</strong>, <strong>NO</strong> means add <strong>0</strong> or substract -425;</p>
  
  <p>HTML:</p>
  
  <pre><code>&lt;input id="priceTag" name="priceTag" type="text" value="400"&gt;
  
  &lt;select id="designChoice" name="designChoice"&gt;
       &lt;option&gt;Choose--&lt;/option&gt;
       &lt;option value="yes"&gt;yes&lt;/option&gt;
       &lt;option value="no"&gt;no&lt;/option&gt;
  &lt;/select&gt;
  </code></pre>
  
  <p>jQuery:</p>
  
  <pre><code>jQuery(document).ready(function($){
  
      var priceTag = $('#priceTag');        
  
      $('#designChoice').on('change', function(){
  
          if($(this).val()==='yes'){              
            /* Here i want to add + 425 to the */
  
          }else{
            /* Here I want to add nothing to the input or substract -425 */   
          }
  
      });
  });
  </code></pre>
  
  <p>What i've tried:</p>
  
  <pre><code>priceTag.val(+ 425);
  /* And more of this type of wrong code ;-P */
  </code></pre>
  
  <p>I've tried to look up existing examples but i didn't find many examples so thanks in advance for the answer!</p>