Larry's Auto Body Repair Shop had revenues that averaged $60,000 per week in April and $50,000 per week in May. During both months, the shop employed six full-time (40 hours week) workers. In April the firm also had four part-time workers working 25 hours per week, but in May there were only two part-time workers and they only worked 10 hours per week. What is the percentage change in labor productivity from April to May for Larry's Auto Body Repair?