You want to retire with at least $1,500,000 in the bank and figure you will continue working for another 40 years. After all of your expenses you can only afford to save $250 a month. What is the...

You want to retire with at least $1,500,000 in the bank and figure you will continue working for another 40 years. After all of your expenses you can only afford to save $250 a month.  What is the lowest interest rate you will need to earn to ensure you meet your retirement goal?