How to textbox key press event inside repeater control in ASP .NET

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ASP

Javascript

.NET

Dear Friends,

I have designed the repeater control with textbox. i need to multiply two textbox columns when i key press in the textbox and show in 3rd column.

What I have tried:

I put like this:
var qty = document.getElementById("<%=txtQty.ClientID%>").value;
var rate = document.getElementById("<%=txtRate.ClientID%>").value;
var amt = document.getElementById("<%=txtAmt.ClientID%>").value;
amt= qty * rate;

its shows an error: The name 'txtQty' does not exist in the current context