var aNumFields = new Array("Summe", "MwSt");
myAverageFunction(aNumFields);

function myAverageFunction(aNumFields)
{
// n = number of fields that have a numerical value
var n=0, sum = 0;
for ( var i=0; i<aNumFields.length; i++) {
var v = this.getField(aNumFields[i]).value;
if ( v != "" ) {
n++;
sum += v;
}
}
if ( n == 0 ) event.value = "";
else event.value = sum/n;
}