font-size: 30px; }
.math-container { margin-top: 10px; margin-bottom: 20px; }
</style>

<script>
var DIGITS = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9];
var MATH_ACTION = ['-', '+', 'x'];
var genProblem = function(){
  d3.select('#math-problem').html('');
  d3.select('#math-answer')[0][0].value = '';

  var digitOne = core.sample(DIGITS);
  var digitTwo = core.sample(DIGITS);
  var action = core.sample(MATH_ACTION);


  // division can be too difficult since there's a good chance
  // it'll almost always result in a n