<input type="text" id="first">
<input type="text" id="second">
<input type="text" id="third">
<input type="button" value="Вычислить" onclick="count();">
<span id="result"></span>
<script>
function count(){
  var a = parseFloat($("#first").val()),
      b = parseFloat($("#second").val()),
      c = parseFloat($("#third").val());
  $("#result").html(((a-b)/c*100));
}
</script>

$("#result").html((a-b)/a*(100-c*0,5));