<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8" />
<script type="text/javascript" src="//code.jquery.com/jquery-latest.min.js"></script>
<script type="text/javascript">
$(function() {
$("input#checkbox").on("click", function() {
var checkbox = $("input#checkbox"), textbox = $("input#textbox");
if(checkbox.is(":checked")) {
textbox.val(Number(textbox.val()) + 50);
}
});
});
</script>
</head>
<body>
<form method="post">
<input type="checkbox" id="checkbox" name="checkbox" />
<input type="text" id="textbox" name="textbox" />
</form>
</body>
</html>