Bob just retired, and wants to move his $500,000 retirement into safer investments.  He has found a very safe municipal bond fund paying 3% interest, and riskier corporate bond fund paying 6% interest.  He wants to invest as little as possible in the riskier account, but needs to earn $21,150 per year to live on.

Set up a system of equations that would allow Bob to determine how much to invest in each account.  Be sure to define your variables clearly.  You do not need to solve the problem, just set it up.