Tryit Editor v2.9

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<!DOCTYPE html>
<html>
<head>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.12.2/jquery.min.js"></script>
<script>
$(document).ready(function(){
    $("#btn1").click(function(){
        $("p:last").css("background-color", "red");
    });
    $("#btn2").click(function(){
        $("p:last-child").css("background-color", "yellow");
    });
});
</script>
</head>
<body>

<button id="btn1">:last</button>
<button id="btn2">:last-child</button><br><br>

<div style="border:1px solid">
  <p>The first paragraph in div.</p>
  <p>The last paragraph in the div.</p>
</div><br>

<div style="border:1px solid">
  <p>The first paragraph in another div.</p>
  <p>The last paragraph in another div.</p>
</div>

</body>
</html>