A retail store had sales of $45,000 in April and $56,000 in May. The store employs 8 full time workers (they work a 40 hour work week). In April the store also had seven part-time workers at 10th hours per week, and in May the store had nine part-timers at 15 hours per week (assume four weeks in each month). Using sales dollars as the measures of output, what is the percentage change in productivity from April to May?