Setting a class works like setting an ID. However, classes aren't unique, so we can set the same class for multiple elements.

Add the grey-element class to the last element of the list.

<html>
<head>
<style>
.grey-element{
background-color: lightgrey;}
</style>
</head>
<body>
  <ol>
   TO DO:
   <li class="grey-element">Floss</li>
   <li>Feed Cat</li>
   <li class="grey-element">Nap</li>
   <li>Plan holiday</li>
   <li class="grey-element">Order tea</li>
  </ol>
</body>
</html>]]>

Make sure to assemble class = "grey-element".