Francisco is planning to invest up to $28,000 in corporate and municipal bonds. The least he is allowed to invest in corporate bonds is $7,000, and he does not want to invest more than $27,000 in corporate bonds. He also does not want to invest more than $16,000 in municipal bonds. The interest is 4% on corporate bonds and 3 1/2% on municipal bonds. This is simple interest for one year. How much should he invest in each type of bond in order to maximize his income? What is the maximum income?