พิมพ์เลขลง text box ในกริดวิว แล้วให้คำนวณมาใส่อีกช่องทำอย่างไรค่ะ Asp.net

  <asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="False" 
                    DataSourceID="SqlDataSource1" Width="1140px" 
        DataKeyNames="id_core" BackColor="White" 
                              BorderColor="#CCCCCC" BorderStyle="None" 
        BorderWidth="1px" CellPadding="3" Height="189px" Font-Size="Medium">
                    <RowStyle ForeColor="#000066" />
                    <Columns>
                      <asp:TemplateField> 
<ItemTemplate>
<%# Databinder.Eval(Container,"DataItemIndex") + 1 %>
</ItemTemplate>
</asp:TemplateField>
                        <asp:TemplateField HeaderText="Core Competency (สมรรถนะความสามารถหลัก)" 
                            SortExpression="core">
                            <EditItemTemplate>
                                <asp:TextBox ID="TextBox1" runat="server" Text='<%# Bind("core") %>'></asp:TextBox>
                            </EditItemTemplate>
                            <ItemTemplate>
                                <asp:Label ID="Label2" runat="server" Text='<%# Bind("core") %>'></asp:Label>
                            </ItemTemplate>
                        </asp:TemplateField>
                        <asp:CommandField SelectText="รายละเอียด" ShowSelectButton="True" />

                        <asp:TemplateField HeaderText="id_core" SortExpression="id_core" 
                            Visible="False">
                        
                            <ItemTemplate>
                                <asp:Label ID="Label11" runat="server" Text='<%# Bind("id_core") %>'></asp:Label>
                            </ItemTemplate>
                        </asp:TemplateField>

                          <asp:TemplateField  HeaderText="1 ">
            <ItemTemplate>
                <asp:TextBox ID="txt1" runat="server" Width="40px" Text = "50" ></asp:TextBox>
            </ItemTemplate>
        </asp:TemplateField>
         <asp:TemplateField  HeaderText="2">
            <ItemTemplate>
                <asp:TextBox ID="txt2" runat="server" Width="40px" ></asp:TextBox>
            </ItemTemplate>
        </asp:TemplateField>
        <asp:TemplateField  HeaderText="3">
            <ItemTemplate>
                <asp:TextBox ID="txt3" runat="server" Width="40px" ></asp:TextBox>
            </ItemTemplate>
        </asp:TemplateField>
        <asp:TemplateField  HeaderText="4">
            <ItemTemplate>
                <asp:TextBox ID="txt4" runat="server" Width="40px"></asp:TextBox>
            </ItemTemplate>
        </asp:TemplateField>
        <asp:TemplateField  HeaderText="5">
            <ItemTemplate>
                <asp:TextBox ID="txt5" runat="server" Width="40px" ></asp:TextBox>
            </ItemTemplate>
        </asp:TemplateField>
        <asp:TemplateField  HeaderText="เฉลี่ย">
            <ItemTemplate>
                <asp:TextBox ID="txtavg" runat="server" Width="40px" Enabled = "false"  ></asp:TextBox>
            </ItemTemplate>
        </asp:TemplateField>




 Protected Sub Button4_Click(ByVal sender As Object, ByVal e As EventArgs) Handles Button4.Click
        Dim txta As TextBox
        Dim txt1a, txt2a, txt3a, txt4a, txt5a, txtavga As TextBox

        Dim i As Integer


        'For i = 0 To GridView1.Rows.Count - 1

        txt1a = CType(GridView1.Rows(1).FindControl("txt1"), TextBox)
        txt2a = CType(GridView1.Rows(1).FindControl("txt2"), TextBox)
        txt3a = CType(GridView1.Rows(1).FindControl("txt3"), TextBox)
        txt4a = CType(GridView1.Rows(1).FindControl("txt4"), TextBox)
        txt5a = CType(GridView1.Rows(1).FindControl("txt5"), TextBox)
        txtavga = CType(GridView1.Rows(1).FindControl("txtavg"), TextBox)

        txt1.Text = txt1a.Text
        txt2.Text = txt2a.Text
        txt3.Text = txt3a.Text

        txt4.Text = txt4a.Text
        txt5.Text = txt5a.Text


        'Next
    End Sub