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You should take advantage of existing classes to use as your root class. NSObject is a common root class.
Fraction is the child or subclass.
Whenever a new class is defined it inherits certain things from the superclass including all non-private methods and instance variables.
Instance variables that are accessed directly by a subclass must be declared in the interface section, and not in the implementation section. Instance variables declared or synthesized in the implementation section are PRIVATE instance variables and are not directly accessible by subclasses.
Rules to choose a message (method) to apply to an object
First, the class to which the object belongs is checked. Then the parent class, and then the next parent class, and so on. If you don't find the method a warning message is generated.