maximize interest income?

Felix and Maria Strom sold some property and decided to invest $65,000 in low-risk government bonds and in higher-risk corporate bonds. Their investment counselor suggested government bonds that paid 4.4% interest and corporate bonds that paid 6.75% interest. Felix and Maria wanted to be sure their investment included a balance of both bond types, so they instructed the investment counselor to invest at least $15,000 in government bonds and no more than $24,000 in corporate bonds. They wanted the maximum return on their total investment. Find the amount invested in each type of bond that will maximize interest income.