$5.$ A $60/40$ participation plan is used in a plant $($base rate is guaranteed and operator receives $60$ percent of proportional gain after exceeding $100$ percent$).$ The established time value on a certain job is $0\mathrm{.}75$min$,$ and the base rate is $ $8\mathrm{.}80.$ What is the direct labor cost per piece when operator efficiency is as follows? a$.50$ percent of standard b$.80$ percent of standard c$.100$ percent of standard d$.120$ percent of standard e$.160$ percent of standard