Try this
<asp:Content ID="Content1" ContentPlaceHolderID="head" runat="Server">
In this Section Like Below
</asp:Content>
<asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder1" runat="Server">
</asp:Content>
<%@ Page Language="C#" MasterPageFile="~/demo.master" AutoEventWireup="true" CodeFile="Office.aspx.cs"
Inherits="Office" %>
<asp:Content ID="Content1" ContentPlaceHolderID="head" runat="Server">
<script type="text/javascript">
function setAverage() {
var a = document.getElementById("txt1").value;
var b = document.getElementById("txt2").value;
var c = document.getElementById("txt3").value;
var d = document.getElementById("dropdown").value;
a = (a == "") ? 0 : a;
b = (b == "") ? 0 : b;
c = (c== "") ? 0 : c;
d = (d == "") ? 0 : d;
var sum = parseInt(a) + parseInt(b) + parseInt(c);
var substract = sum + parseInt(d);
document.getElementById("txt4").value = sum;
document.getElementById("txt5").value = substract;
}
</script>
</asp:Content>
<asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder1" runat="Server">
</asp:Content>