Final answer:
Mary's investment rate can be found using the Present Value of an Annuity formula, which requires the initial investment and series of annuity payments known, but cannot be calculated exactly without the formula or a financial calculator.
Step-by-step explanation:
To calculate the rate of interest that Mary's investment is earning, we can use the formula for the Present Value of an Annuity. In this scenario, Mary's investment is the present value, and she receives a series of $1000 payments over 15 years as an annuity.
Although the exact formula isn't provided, it typically involves solving for the interest rate that makes the present value of the expected annuity payments equal the initial investment. However, without the explicit formula or a financial calculator, it's not possible to calculate the exact interest rate from the information given.
The question provided is only related in demonstrating the concept of present value and compound interest. To solve Mary's scenario correctly, a financial calculator or appropriate software to find the interest rate would be used, often by trial and error with different rates until the equation balances.