A manufacturer produces two types of computer monitors: monochrome and color. Past sales experience shows that at least twice as many monochrome monitors are sold as color monitors. The manufacturing plant is capable of producing 12 monitors per day. A profit of $50 is earned on each monochrome monitor sold, and $75 is earned on each color monitor sold. How many of each monitor should be sold to maximize the manufacturer's profit? (Solve graphically or algebraically).