ddro3doff,
на всякий случай ...
<!DOCTYPE html>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<style type="text/css">
#res {
font-weight: bold;
}
</style>
</head>
<body>
<a class="content_toggle" href="#" onclick="mufunk(event,this);" data-filter="fikl">
Фиркл </a>
<table id="fikl">
<tr>
<td>2</td>
</tr>
<tr>
<td>7</td>
</tr>
</table>
<div id="res" style=""></div>
<script>
function mufunk(event, element) {
event.preventDefault();
let id = `#${element.dataset.filter}`, sum = 0,
{ length } = tds = document.querySelectorAll(`${id} td`);
tds.forEach(({ textContent }) => sum += +textContent);
document.querySelector('#res').innerHTML = (sum / length).toFixed(2)
}
</script>
</body>
</html>