pava467,
<!DOCTYPE html>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<style type="text/css">
</style>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.12.4/jquery.min.js"></script>
<script>
$(function() {
$('.cena').text(function(index, num) {
return num * 20
});
});
</script>
</head>
<body>
<span class="cena">5</span>
<span class="cena">6</span>
<span class="cena">7</span>
</body>
</html>