Something

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

// HIER MEINE FUNKTIONEN


function myAverageFunction(aNumFields)
{
var sum = ValueToText(this.getField("Summe").value);
event.value = sum/n;
}