javascript : Random()???

var answer = new Array();

function dobbel(nr) {
 return Math.ceil(Math.random() * nr);
 };

function showSum(i,a,x,b) {
 var f = document.tableForm;
 f.one[i].value = a;
 f.times[i].value = x;
 f.two[i].value = b;
 f.equals[i].value = '';
 f.equals[i].style.background = '#fff';
 };

function setTables() {
 var a, b;
 for (var i = 0; i < 10; i++) {
  var method = dobbel(4); // + - x :
  switch (method) {
   case 1:
    a = dobbel(10);
    b = dobbel(10);
    showSum(i,a,'+',b);
    answer[i] = a + b;
    break;
   case 2:
    a = dobbel(10);
    b = dobbel(10);
    showSum(i,a+b,'-',a);
    answer[i] = b;
    break;
   case 3:
    a = dobbel(9) + 1;
    b = dobbel(9) + 1;
    showSum(i,a,'x',b);
    answer[i] = a * b;
    break;
   case 4:
    a = dobbel(9) + 1;
    b = dobbel(9) + 1;
    showSum(i,a*b,':',a);
    answer[i] = b;
    break;
   };
  };
 };

function checkAnswers() {
 var f = document.tableForm, color;
 for (var i = 0; i < 10; i++) {
  if (parseInt(f.equals[i].value) == answer[i]) {
   color = '#dfd';
   } else {
   color = '#fdd';
   };
  f.equals[i].style.background = color;
  };
 };