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"Null" is only valid for object. double is not an object, it is a primitive. The "default" value for a double is 0. I suppose if you really really wanted to return null, you could turn the return...

SavingsAccount probably shouldn't be abstract. You cannot initialize abstract classes. If anything, Account should be abstract and should contain an abstract add_monthly_interest() method. But, that...

While not knowing the exact lines you are having errors, I can tell you that the Account class doesn't have an add_monthly_interest method, so you cannot invoke that method on an Account object. You...