Imagine Cup Developer Camp TipCalculator sample code showing how to share a PCL and other common code between Windows, Windows Phone, iPhone and Android.
Purposely did not use MVVM as it's part of a presentation that shows how to build it with copy paste then go to Portable Class Library.
To see an MVVM sample of tipCalculator see http://www.codeproject.com/Articles/566644/MvvmCross-TipCalc-A-Recap which is an MVVMCross implementation. This sample was inspired from that.