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answer:

We will need to add an "insert multiply" method that will add "X" to the history in the simple calculator impl class and preferably extend the simple calculator interface. Moreover we will need to modify the "insert equals" method so it will be able to process multiplication. then we will need to change the UI and add the "X" button. Lastly, we will need to connect the button and the "insert multiply" in Main Activity.

In terms of testing we will need to test the basic functionality of "insert multiply" that is, checking that "X" is added to the history and checking that "insert equals" calculates the given formula correctly (both syntactically and semantically). Afterwards we will need to verify that the "X" button works properly through a main activity test and lastly we will preform some flow tests combining the "X" button to make sure that our app works properly.