average.rb

def average(*numbers)
	((numbers.inject {|sum, element| sum + element}) / numbers.length).to_f
end

c++way.cpp

#include <stdarg.h>
 
double average(int count, ...)
{
    va_list ap;
    int j;
    double tot = 0;
    va_start(ap, count);
    for(j=0; j<count; j++)
        tot+=va_arg(ap, double);
    va_end(ap);
    return tot/count;
}

csharpway.cs

double Avg(params double[] a)
{
   return a.Sum() / a.Length;
}

javascriptway.js

function average(){
   numbers=average.arguments;
   sum=0;
   for(var i=0; i<numbers.length; i++){
      sum+=numbers[i]
   };
   return sum/numbers.length;
}

javaway.java

public static double average(int... numbers) {
	// Needs Java 1.5
	double count = 0;
	double sum = 0;
	for(int num: numbers) {
		count++;
		sum += num;
	}
	return sum / count;
}

lispway.lisp

(defun average (&rest numbers)
  (/ (apply #'+ numbers)(length numbers)))

luaway.lua

function average (...)
  sum = 0
  for _,n in ipairs({...}) do sum = sum + n end
  return sum / #{...}
end

perlway.pl

sub Average
{
         return sum(@_)/scalar(@_);
}

phpway.php

<?php
function average() {
	return floatval(array_sum(func_get_args())/func_num_args());
}

pythonway.py

def average(*numbers):
   return float(sum(numbers))/len(numbers)