Strange. The 119th answer is fine for me, but the 120th breaks it. I revised the script to not rely on interval multiplication, which can be flighty due to inconsistent latencies, but to rather keep a counter and cancel the interval after 119 solves.

This is the script I used, but yours blocks also on 119 for me.

    var answer = function() {
      var q = $("#question").text().replace(/x|÷|=/g, function(m) { return {'x':'*','÷':'/','=':''}[m]});
      $("#answer").val(eval(q)).keyup();
    };
    $('#question').bind("DOMSubtreeModified", function(){
      setTimeout(answer, 200);
    });
    answer();

Don't have much time to search why :-)