<s>[INST] This problem appeared in the final exam of a well-known data science boot camp. In this problem, we can use Python to create a class that represents a bank account for an investment that compounds interest monthly. We are given the starting principal balance, a nominal annual interest rate, the time the money is invested for in years, and a monthly deposit. We create a new class called `MonthlyInvestment` which is initialized with the principal balance, interest rate, and investment duration. We then create three instance methods: a private method `_calculate_multiplier()` that calculates the monthly interest rate, a method `get_balance()` that returns the current balance, and a method `add_month()` that simulates a month's worth of investment by adding the monthly deposit and calculating interest.[/INST]