Answer:
both must implement a method called calculate and will not be able to access the instance variable.
Explanation:
Based on the information provided within the question it can be said that any concrete subclass that extends class Foo both must implement a method called calculate and will not be able to access the instance variable. As shown in the code above the method inside the Foo subclass needs to exist outside of it in order for it to be called, and since the a variable is an instance variable (meaning it is only accessible by code inside the function) it cannot be used by extending classes.
