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How to Calculate GST by input value in text box Jquery

<!DOCTYPE html>
<html>
<body>
    <head>
        <script src="https://ajax.googleapis.com/ajax/libs/jquery/1.8.2/jquery.min.js" type="text/javascript"></script>
    </head>
    <h1>GST Calculation</h1>
    <form method="post" action="">
        <input type="hidden" name="mode" value="PinRequest" />
        <label><b>Fee</b></label>
        <input class="w3-input w3-border" name="pages" id="pages" type="text"><br><br>
        <label><b>Total Amount</b></label>
        <input class="w3-input w3-border" name="tot_amount" id="tot_amount" type="text">
        <input type="submit" name="save" value="submit">
    </form>
    <script>
        $(document).ready(function() {
            $('#pages').keyup(function(ev) {
                var total = $('#pages').val() * 1;
                var tot_price = total + (total * 0.18);
                var divobj = document.getElementById('tot_amount');
                divobj.value = tot_price;
            });
        });
    </script>
    <body>
</html>

Run it yourself

Demo