codice:
  <form name="base">
    <input name="op1" value="0" type="text" onblur="add(this.form.op1.value, this.form.op2.value, this.form.ans);" /> + 
    <input name="op2" value="0" type="text" onblur="add(this.form.op1.value, this.form.op2.value, this.form.ans);" /> = 
    <input name="ans" value="0" type="text" /> 
    <input type="button" value=" [addiziona] " onclick="add(this.form.op1.value, this.form.op2.value, this.form.ans);" />
  </form>

  <script type="text/javascript">
  <!--

    function add (source1, source2, answer)
    {
      source1 = parseFloat(source1);
      source2 = parseFloat(source2);

      if (!answer || isNaN(source1) || isNaN(source2))
        return;

      answer.value = Math.round ((source1+source2)*100) / 100;
    }

  //-->
  </script>
si è lo stesso: basta solo avere cura delle parentesi per favorire la precedenza all'addizione.