George and Susannah have $350,000 they want to invest for retirement. They can invest in bonds (6% return), mutual funds (9% return), or stocks (15% return). The stocks are far riskier so they want to invest at least twice as much in bonds and mutual funds combined as they do in stocks. They also want to invest at least 20% of their investment in bonds. How much should they invest in each in order to maximize their returns?