A company has introduced a process improvement that reduces processing time for each unit, so that output is increased by $30\%$ with less material, but two additional workers are required. Under the old process, six workers could produce $70$ units per hour in total. Labour costs are $$12/$hour$,$ and material input was previously $$16/$unit$.$ For the new process, material is now $$10/$unit$.$ Overhead is charged at $1\mathrm{.}6$ times direct labour cost. Finished units sell for $$31$ each. What increase in productivity is associated with the process improvement?