on-integer, so let's
  // avoid those kinds of problems.
  if(action === '+'){
    var expectedAnswer = digitOne + digitTwo;
  } else if(action === '-'){
    var expectedAnswer = digitOne - digitTwo;
  } else if(action === 'x'){
    var expectedAnswer = digitOne * digitTwo;
  }

  d3.select('#math-problem').html(digitOne + ' ' + action + ' ' + digitTwo + ' = ');

  d3.select('#subbtn').on('click', function(){
    var userAnswer = d3.select('#math-answer')[0][0].value;
    var r = userAnswer === expectedAnsw