Kevin is an auto mechanic. He spends 2 hours when he replaces the shocks on a car and 2 hours when he replaces the brakes. He works no more than 54 hours a week. He routinely completes at least 4 shocks replacements and 6 break replacements a week. If he charges $500 for labor replacing shock and $300 in labor for replacing breaks, how many jobs of each type should he complete a week to maximize his income?