Calculate Percentage based on TextBox value using C# and VB.Net in ASP.Net

after getting Sub Total from gridview I want to give discount of 7% by using update panel. and want to show the amount in the Total Bill text box like this. 

for example 

Sub Total = 500

discount = 7 %

Total Bill = 465

i have written code but it is not working how to fix it. my code is  

<script type="text/javascript" src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js"></script>
<script type="text/javascript">
        $(function () {
    var temprow = $('[id*=GridView2] table tbody').find(".emptyTd");
    $('[id*=cbCheck]').change(function () {
        var checkbox = $(this);
        if ($(this).is(":checked")) {
            var row = $(this).parent().closest('tr');
            var temp = $(row).clone(true);
            $('[id*=GridView1] tbody').append(temp);
            $('[id*=GridView2] table tbody').find(".emptyTd").remove();
            $(row).find("td:first").remove();
            $('[id*=GridView2] table tbody').append(row);
        }
        else {
            $('[id*=GridView2] table tbody tr').each(function () {
                if ($(this).find("td:first").html() == $(checkbox).parent().closest('tr').find("td:nth-child(2)").html()) {
                    $(this).remove();
                    if ($('[id*=GridView2] table tbody').has('td').length == 0) {
                        $('[id*=GridView2] table tbody').append(temprow);
                    }
                }
            });
        }
    });
});       
</script>
<script type="text/javascript" src="https://ajax.googleapis.com/ajax/libs/jquery/1.8.3/jquery.min.js"></script>
<script type="text/javascript">
    $(function () {
        $("[id*=txtQuantity]").val("0");
    });
    $("body").on("change keyup", "[id*=txtQuantity]", function () {
        //Check whether Quantity value is valid Float number.
        var quantity = parseFloat($.trim($(this).val()));
        if (isNaN(quantity)) {
            quantity = 0;
        }

        //Update the Quantity TextBox.
        $(this).val(quantity);

        //Calculate and update Row Total.
        var row = $(this).closest("tr");
        $("[id*=lblTotal]", row).html(parseFloat($(".price", row).html()) * parseFloat($(this).val()));

        //Calculate and update Grand Total.
        var grandTotal = 0;
        $("[id*=lblTotal]").each(function () {
            grandTotal = grandTotal + parseFloat($(this).html());
        });
        $("[id*=lblGrandTotal]").html(grandTotal.toString());
    });
</script>


  <div>
    <asp:GridView ID="GridView1" HeaderStyle-BackColor="#3AC0F2" HeaderStyle-ForeColor="White" Class="table table-striped table-bordered table-hover"
    runat="server" AutoGenerateColumns="false" ShowFooter="false" >
    <Columns>
        <asp:TemplateField>
            <ItemTemplate>
                <asp:CheckBox ID="cbCheck" runat="server" />
            </ItemTemplate>
        </asp:TemplateField>
        <asp:BoundField DataField="ItemCode" HeaderText="Code" ItemStyle-Width="200" />
        <asp:BoundField DataField="ItemName" HeaderText="Product" ItemStyle-Width="200" />
        <asp:BoundField DataField="ItemSize" HeaderText="Weight / Size" ItemStyle-Width="200" />
        <asp:BoundField DataField="Qty" HeaderText="Qty" ItemStyle-Width="150" />
         <asp:BoundField DataField="Rate" HeaderText="Price" ItemStyle-CssClass="price" />
        <asp:TemplateField HeaderText="Quantity">
            <ItemTemplate>
                <asp:TextBox ID="txtQuantity" runat="server" class="form-control"></asp:TextBox>
            </ItemTemplate>
        </asp:TemplateField>
        <asp:TemplateField HeaderText="Total">
            <ItemTemplate>
                <asp:Label ID="lblTotal" runat="server" Text="0" class="control-label"></asp:Label>
            </ItemTemplate>
        </asp:TemplateField>
    </Columns>
</asp:GridView>
<br />
<br />
<asp:GridView ID="GridView2" HeaderStyle-BackColor="#3AC0F2" HeaderStyle-ForeColor="White" Class="table table-striped table-bordered table-hover"
    runat="server" AutoGenerateColumns="false">
    <EmptyDataTemplate>
        <table border="1" cellpadding="0" cellspacing="0" >
            <tr style="color: White; background-color: #3AC0F2;">
                <th scope="col">
                    Code
                </th>
                <th scope="col">
                    Product
                </th>
                <th scope="col">
                    Weight/Size
                </th>
                <th scope="col">
                    Qty
                </th>
                <th scope="col">
                    Price
                </th>
                <th scope="col">
                    Count
                </th>
                <th scope="col">
                   Total
                </th>
            </tr>
            <tr class="emptyTd">
                <td style="width: 30px; text-align: center" colspan="7">
                    No Records found
                </td>
            </tr>
        </table>
    </EmptyDataTemplate>
</asp:GridView>
<br />
<label class="control-label" style="margin-top: 10px;color:Green; font-size:large; font-weight:bolder; font-style:oblique;">Sub Total:</label>
<asp:Label ID="lblGrandTotal" runat="server" Text="0" class="control-label" style="margin-top: 10px; color:Red; font-size:large; font-weight:bolder; font-style:italic;" ></asp:Label>    

    <div class="col-sm-3">

    <asp:UpdatePanel ID="SCPanelDis" runat="server" UpdateMode="Conditional" ChildrenAsTriggers="false">
            <ContentTemplate>                              
                <label class="control-label" style="margin-top: 10px;">
                    Discount</label>
                <asp:TextBox ID="txtDis" runat="server" class="form-control" Font-Size="Large" Font-Bold="true" placeholder="0"
                    AutoPostBack="true" OnTextChanged="UpdateDis_Text"></asp:TextBox>
                <label class="control-label" style="margin-top: 10px;">
                    Total Bill</label>
                <asp:TextBox ID="txtGTotal" runat="server" class="form-control" Font-Size="Large" Font-Bold="true" placeholder="0"
                    ReadOnly="True"></asp:TextBox>
            </ContentTemplate>
        </asp:UpdatePanel>        
    </div>
<div class="col-sm-3 ">
        <asp:UpdatePanel ID="SCPanel" runat="server" UpdateMode="Conditional" ChildrenAsTriggers="false">
            <ContentTemplate>                              
                <label class="control-label" style="margin-top: 10px;">
                    Paid</label>
                <asp:TextBox ID="txtPaid" runat="server" class="form-control" Font-Size="Large" Font-Bold="true" placeholder="0"
                    AutoPostBack="true" OnTextChanged="Update_Text"></asp:TextBox>
                <label class="control-label" style="margin-top: 10px;">
                    Change</label>
                <asp:TextBox ID="txtRemBal" runat="server" class="form-control" Font-Size="Large" Font-Bold="true" placeholder="0"
                    ReadOnly="True"></asp:TextBox>
            </ContentTemplate>
        </asp:UpdatePanel><br/>        
        <asp:Button ID="btnSale" runat="server" Text="Make Sale" ControlStyle-CssClass="btn btn-success" />
    </div>
</div>

    
    protected void UpdateDis_Text(object sender, EventArgs e)
    {
        int value = Convert.ToInt32(lblGrandTotal.Text);
        double percent = Convert.ToDouble(txtDis.Text);
        txtGTotal.Text = ((value * percent) / 100 + value).ToString();
        //Response.Write(txtGTotal.Text);
        SCPanelDis.Update();
        
    }
    protected void Update_Text(object sender, EventArgs e)
    {
        int val1 = 0;
        int val2 = 0;
        string Tot = txtGTotal.Text.Trim();
        int.TryParse(txtPaid.Text, out val1);
        int.TryParse(Tot, out val2);
        int I = (val2 - val1);
        txtRemBal.Text = I.ToString();
        SCPanel.Update();
    }