An investment firm can invest in four financial instruments: Trade Credits, Bonds, Gold Stocks, and Construction loans. The income from each investment will be 7%, 11%, 19%, and 15% respectively. The firm has $5 million to invest and they do not want to invest more than $1.6 million in any single instrument. Furthermore, the firm wants at least 20% investment in Gold stocks and no less than 8% in Trade Credits. Set up and solve this LP problem. How much should they invest in each and what will be the return on the entire portfolio?