I have code :
function compute() {
if ($(\'input[name=type]:checked\').val() != undefined) {
var a = $(\'input[name=service_price]\').val();
Your code is correct, except you are adding (concatenating) strings, not adding integers. Just change your code into:
function compute() {
if ( $('input[name=type]:checked').val() != undefined ) {
var a = parseInt($('input[name=service_price]').val());
var b = parseInt($('input[name=modem_price]').val());
var total = a+b;
$('#total_price').val(a+b);
}
}
and this should work.
Here is some working example that updates the sum when the value when checkbox is checked (and if this is checked, the value is also updated when one of the fields is changed): jsfiddle.