jQuery Form Calculation - Works, but strange results -

i have form inputs laid out correctly using id's etc. have deployed little jquery function calculate form variables. in essence want calculate total vat , grand total.

the code example taken http://forum.jquery.com/topic/calculation-on-form-fields , works ok. multiplication/division works vat addition gives strange results.

example data input:
sitefee = 3000
qty = 1
vat = 20

the vat function (a * b * c) /100 calculates ok , gives 600 correct grandtotal comes out @ 3000611.6659999999999 doesn't make lot of sense. looks it's trying concatenate values rather adding them together, again that's not occurring either!.

jquery code:

<script>          $(document).ready(function() {            function compute() {                   var = $('#sitefee').val();                   var b = $('#qty').val();                   var c = $('#vat').val();                   var d = $('#incvat').val();                   var totalvat = (a * b * c) / 100;                   var grandtotal = + d;                      $('#incvat').val(totalvat);                     $('#total').val(grandtotal);                  }                  $('#sitefee, #qty, #sitefeeid, #vat').change(compute);          });     </script> 

note #sitefeeid form variable ajax pull correct values #sitefee shouldn't matter, thought mention it.

any pointers wonderful.

cheers nick

try using parsefloat() or parseint() functions whle performing calculations in javascript becasue + opertaor concatanation in javascript

so var grandtotal = parsefloat(a) + parsefloat(d);