尝试一下（更新以显示ASPX和VB代码，并带有几个静态值进行测试）

Try this (UPDATED TO SHOW ASPX and VB code with a couple of static values for test)

aspx代码

<asp:TextBox ID="txtsurcharges" runat="server" Text="£5" AutoPostBack="true"></asp:TextBox>
    <asp:TextBox ID="txttotal" runat="server" Text="£5"></asp:TextBox>

vb代码

Private Sub txtsurcharges_TextChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles txtsurcharges.TextChanged 
    Dim c As Integer 
    c = CInt(txtsurcharges.Text) + CInt(txttotal.Text) 
    txttotal.Text = c
End Sub

您可以

如果我现在将txtsurcharges更改为10，则txttotal将更改为15。

If I now change txtsurcharges to 10 then txttotal will change to 15.

当然，您需要确保只能在txtsurcharges中键入数字。

You will of course need to make sure that only numerics can be typed into txtsurcharges.