A math professor plans to retire in 20 years. She wishes to have a retirement income of $4000

per month, paid out over a period of 30 years (at the end of that time period, the account will

be totally depleted). Assuming that her 401k account earns an average of 4.5% interest

compounded monthly, what monthly payment should she be making over the next 15 years to

accomplish her goal.