How to multiply dropdown value with in textbox value inside a gridview using javascript

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VB

ASP.NET

Javascript

jQuery

sir, i have a gridview in which i have 3 column:- ID,Number,Amount.

<asp:GridView ID="Gridview1" runat="server" AutoGenerateColumns="false"  DataKeyNames="ID">
                      <Columns>

                          <asp:TemplateField Visible="false">
                              <ItemTemplate>
                                  <asp:Label ID="LblFeeHeadOther" runat="server" Text='<%# Bind("ID") %>'></asp:Label>
                              </ItemTemplate>
                          </asp:TemplateField>

              <asp:TemplateField SortExpression="Amount" HeaderText="n(Amount)"  HeaderStyle-Width="70px">
                              <ItemTemplate>
                                  <asp:DropDownList ID="DdlMultiply" runat="server"   AutoPostBack="true"   OnSelectedIndexChanged="Multiply">
                                  <asp:ListItem Value="1">1</asp:ListItem>
                                  <asp:ListItem Value="2">2</asp:ListItem>
                                  <asp:ListItem Value="3">3</asp:ListItem>
                                  <asp:ListItem Value="4">4</asp:ListItem>
                                  <asp:ListItem Value="5">5</asp:ListItem>
                                  <asp:ListItem Value="6">6</asp:ListItem>
                                  <asp:ListItem Value="7">7</asp:ListItem>
                                  <asp:ListItem Value="8">8</asp:ListItem>
                                  <asp:ListItem Value="9">9</asp:ListItem>
                                  <asp:ListItem Value="10">10</asp:ListItem>
                                  </asp:DropDownList>
                           </ItemTemplate>
                          </asp:TemplateField>

                          <asp:TemplateField SortExpression="Amount" HeaderText="Amount" ItemStyle-HorizontalAlign="Right" HeaderStyle-HorizontalAlign="Right"  HeaderStyle-Width="60px">
                              <ItemTemplate>
                                  <asp:TextBox ID="TxtRecd"  Text='<%# Bind("Amount") %>' runat="server" Width="100px"   ></asp:TextBox>
                            </ItemTemplate>
                          </asp:TemplateField>

                      </Columns>
                  </asp:GridView>

it look like after binding:-

ID -- Number -- Amount
101-- 1 -- 500
102-- 1 -- 700

it want that when i change dropdown then dropdown value multiply with amount and now amount are bind with new value. for example in first row i am select 5 in dropdown list then my gridview look like:-

ID -- Number -- Amount
101-- 5 -- 2500
102-- 1 -- 700

What I have tried:

its done fine in vb.net. but i want to do it with java script.

Protected Sub Multiply(ByVal sender As Object, ByVal e As EventArgs)

Dim row As GridViewRow = TryCast(TryCast(sender, DropDownList).NamingContainer, GridViewRow)
Dim dataKey As String = Me.Gridview1.DataKeys(row.RowIndex).Value.ToString()

Dim ClassesID As String = ViewState("ClassesID")


Dim amount As Integer = ViewState("amount")

Dim n As Integer = CType(GridOtherFee.Rows(row.RowIndex).FindControl("DdlMultiply"), DropDownList).SelectedValue

CType(row.FindControl("TxtRecd"), TextBox).Text = n * amount
End Sub