title: Change observable value on dropdown change Knockout Js
body: <p>I have this dropdown which have options if vehicle is new or used.</p>
  
  <pre><code>&lt;select name="VehicleType" id="vehicleTypeDropdown" data-bind="value: VehicleType"&gt;    
      &lt;option value="New" selected="selected"&gt;Nuevo&lt;/option&gt;
      &lt;option value="Used"&gt;Usado&lt;/option&gt;
  &lt;/select&gt; `
  </code></pre>
  
  <p>And this input:</p>
  
  <pre><code>&lt;input type="text" name="Mileage" data-bind="disable: VehicleType() === 'New',value:  Mileage" class="input-large"&gt; `
  </code></pre>
  
  <p>If the value in the dropdown selected is New the input must be disabled, and if used the input should be enabled, but if I enter a value the observable will grab this value and if I change the dropdown value to new the observable must become zero.</p>