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The first application shows how you can put a black-colored window on the iPhone’s screen, allow for the user to press a button, and then display some text in response to the pressing of that button.
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The second application is more fun! You use the knowledge gained from your first application to build a simple calculator that does operations with fractions. You can use your Fraction class that you worked with earlier in the book, as well as a modified Calculator class. This time, your calculator needs to know how to work with fractions.