I have a XML structure like this : 

```
<?xml version="1.0"?>
<catalog>

   <book id="bk101">
      <title>XML Developer's Guide</title>
      <price>44.95</price>
      <sale-price>4.90</sale-price>
   </book>

   <book id="bk102">
      <title>Midnight Rain</title>
      <price>5.95</price>
      <sale-price>4.90</sale-price>
   </book>

</catalog>

```

I want to select nodes where the sale price is discounted more than 30% from the regular price. How can I do that using XPath?