<script language="JavaScript">
 <!-- hide
s=prompt("Введите n",'0');
n=parseInt(s);
s=prompt("Введите l",'0');
l=parseInt(s);
var m=0
var x=1;
var k=new Array ();
for (i = 0; i < n; i++)
{k[i]=new Array ();
for (j = 0; j < l; j++)
{k[i][j] = i*10 + j;
k[i,j]=prompt("Введите число",""); 





if (k[i,j]%2=0) { var x=x+k[i,j]
m=m+1}
} }

x=x/m

alert(x)


// -->
 </script>

<script>
window.onload = function () {//onload begin
  var x = 2;
  var y = 2;
  var mas = [];

  for (var i = 0; i < x; i++) {
     mas[i] = [];
    for (var j = 0; j < y; j++) {
      mas[i][j] = parseInt(prompt('Введите элемент [' + i + ',' + j + ']'));
      document.write('[' + i + ',' + j + '] = ' + mas[i][j] + '; ');
    }
    document.write('<br>')
  }

  var sum = 0;
  for (var i = 0; i < x; i++) {
    for (var j = 0; j < y; j++) {
      if (mas[i][j] % 2 == 0) {
        sum += mas[i][j];
      }
    }
  }
  document.write('<br><br>s-a = ' + sum/(x*y))

}//onload end
</script>

var res = []
var arr = [
  [10,20,30,40,50],
  [100,200,300,400,500]
]
for (k=0;k<arr[0].length;k++) res.push((arr[0][k] + arr[1][k]) / 2)

for (i=0;i<res.length;i++) alert( res[i] )