jQuery adding 2 numbers from input fields

Adding strings concatenates them:

> "1" + "1"
"11"

You have to parse them into numbers first:

/* parseFloat is used here. 
 * Because of it's not known that 
 * whether the number has fractional places.
 */

var a = parseFloat($('#a').val()),
    b = parseFloat($('#b').val());

Also, you have to get the values from inside of the click handler:

$("submit").on("click", function() {
   var a = parseInt($('#a').val(), 10),
       b = parseInt($('#b').val(), 10);
});

Otherwise, you're using the values of the textboxes from when the page loads.

<script type="text/javascript">
    $(document).ready(function () {
        $('#btnadd').on('click', function () {
            var n1 = parseInt($('#txtn1').val());
            var n2 = parseInt($('#txtn2').val());
            var r = n1 + n2;
            alert("sum of 2 No= " + r);
            return false;
        });
        $('#btnclear').on('click', function () {
            $('#txtn1').val('');
            $('#txtn2').val('');
            $('#txtn1').focus();
            return false;
        });
    });
</script>