Tweaked how numbers are generated.

var cuberoot = document.getElementById("cuberoot");

var result = document.getElementById("result");

var stats = document.getElementById("stats");

var began;

var finished;

var elapsed = 0;

var generated;

var guesses = 0;

var correct = 0;

var began, finished, generated;

var elapsed = 0, guesses = 0, correct = 0;

var generator = [];

Array.prototype.shuffle = function () {

  var i = this.length, val, j;

  if (i > 2) {

    while (i > 0) {

      j = Math.floor(Math.random() * i);

      i--;

      if (i != j) {

        val = this[i];

        this[i] = this[j];

        this[j] = val;

      }

    }

  }

  return this;

}

Array.prototype.random = function() {

  if (this.length == 0) {

    for (var i = 10; i < 100; i++) {

      this.push(i);

      if (i % 10 > 0) {

        this.push(i);

      }

    }

    this.shuffle();

  }

  return this.shift();

}

cuberoot.addEventListener("keydown", function (e) {

  if (e.keyCode === 13) {

function start_round() {

  if (guesses > 0) {

    stats.innerHTML = "Got " + correct + " correct out of " + guesses +

    stats.innerHTML = "Got " + correct + " out of " + guesses +

                      ". Last round took " + humanize_ms(finished - began) +

                      "." + ((correct > 0)

                          ?" Average time of correct guess was " +

                          ?" Avg time of correct guesses was " +

                          humanize_ms(elapsed / correct) + "."

                          :"");

  }

  generated = Math.floor(Math.random() * 90) + 10;

  generated = generator.random();

  cube.innerHTML = " " + Math.pow(generated, 3) + " ";

  began = Date.now();

  cuberoot.value = "";

    elapsed += (finished - began);

    result.innerHTML = "Correct!";

  } else {

    result.innerHTML = 'Not correct! <span style="white-space:nowrap;">' +

    generator.push(generated, generated);

    generator.shuffle();

    result.innerHTML = 'Incorrect! <span style="white-space:nowrap;">' +

      '<span style="font-size:larger;">&#8731;</span>' +

      '<span style="text-decoration:overline;">&nbsp;' +

      Math.pow(generated, 3) +