As a $25-$year$-$old, I have set a goal of being a millionaire by the time I am $60.$ At the age of $25$ I have invested $$25,000$ in the stock market and historically the stock market has returned somewhere in the neighborhood of $11\%.$ How much am I going to have to add monthly $($payments$)$ to my investment in order to achieve my $$1$ million goal over $35$ years $(420$ months$)?$