Lin Shou Shan (superlevin), a Delphi developer from Taiwan, has a blog post tutorial up where he builds a calculator app demo with Delphi XE6 Firemonkey. It should also work in Delphi XE5 and AppMethod. The blog post is in Chinese but you can use Google Chrome to automatically translate it. The calculator demo supports addition, subtraction, multiplication, and division. It is made up of a TEdit control aligned to the top of the screen and then a bunch of standard TButton controls for the rest of the interface. There isn’t any platform dependent code in this demo so it should work cross platform on Android, IOS, Windows, and OSX. I like how the operator function is built in the demo to store the operator so that when you press the equal sign it performs the math operation. The whole demo is written in only 116 lines of code to implement a basic calculator and that seems really compact to me.. The source code for the demo app is up on Github.