If you want to show it to the user before the form is submitted, then you have to use javascript.
You would add an onblur event to each of the fields and call a javascript function. The function would be something like:
<script>
function showAverage() {
var average = 0;
var field1 = document.getElementById("field1");
var field2 = document.getElementById("field2");
var field3 = document.getElementById("field3");
var field4 = document.getElementById("field4");
var field5 = document.getElementById("field5");
if (!isNaN(parseFloat(field1.value))) {
average += parseFloat(field1.value);
}
if (!isNaN(parseFloat(field2.value))) {
average += parseFloat(field2.value);
}
if (!isNaN(parseFloat(field3.value))) {
average += parseFloat(field3.value);
}
if (!isNaN(parseFloat(field4.value))) {
average += parseFloat(field4.value);
}
if (!isNaN(parseFloat(field5.value))) {
average += parseFloat(field5.value);
}
average = avarage/5;
average = Math.round(average*100)/100;
document.getElementByID("displayAverage").innerHTML = String(average);
}
</script>
This assumes you have fields with id="field1" thru "field5" and a span with id="displayAverage". Then in each field you add onblur="showAverage()"