Kevin is an auto mechanic. He spends 3 hours when he replaces the shocks on a car and 2 hours when he replaces the brakes. He works no more then 42 hours a week. He routinely completes at least 4 shocks replacements and 6 brake replacements a week. If he charges $400 for labor replacing shocks and $300 for replacing brakes, how many jobs of each type should he complete a week to maximize his income?