$16\mathrm{.}13.$ Backyard Equipment manufactures garden tillers, leaf blowers, and lawn mowers for home use in a mixed$-$model assembly line. Annual demand is estimated as $4500$ for garden tillers, $3000$ for leaf blowers, and $9000$ for lawn mowers. The plant is currently operating one shift at an overtime schedule of $25$ days per month. Backyard is pursuing the lean principles of uniform production and mixed$-$model assembly.