w20.js

var w20 = function(x, y) {
  var wurf = Math.floor(Math.random() * 20) + 1;
  var mustbe = Math.floor(20 - 20 * x / y) + 1;
  return {wurf: wurf, mustbe: mustbe, result: wurf >= mustbe};
};

for(var y = 1; y <= 20; y++) {
  for(var x = 1; x <= y; x++) {
    var result;
    do {
      result = w20(x, y);
      console.log(x + "/" + y + ": " + result.wurf + ", " + result.mustbe + ", " + result.result);
    } while (result.result === false);
  }
}