A bond mutual fund invests only in bonds rated A, AA, or AAA. The fund's policy requires that no more than 65% of the total investment be in A and AA bonds and at least 50% of the total investment be in AA and AAA bonds. The expected return on the bonds is 7.2% for the A bonds, 6.8% for the AA bonds, and 6.5% for the AAA bonds. Find the percentage of the investment in each category that maximizes returns.