The Telerik.Quickstart namespace is part of the demo web site app only for the purposes of the demos.
If you do want to use it, you can find its code in the App_Code folder of the demo site you can download from your account: Go to https://www.telerik.com/account/product-download?product=RCAJAX and download Telerik_UI_for_ASP.NET_AJAX_20xx_x_xxx_Demos_[Dev|Trial].msi.
Alternatively, the Isolate this demo as a stand-alone application page shows how to remove it from the demos. This page is available as a link on every demo.
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