b$3.$ Consider a process consisting of three resources. Assume there exists unlimited demand for the product.

a$.$ Resource $1$ has a processing time of $6$ minutes per unit.

The cost of direct labor is the total wages per unit of time divided by the flow rate.

The flow rate is $(1/6)$ parts per minutes

Total wages per unit of time $=$ $$12*3/60=$ $$12/20$ per minute

So$,$ cost of direct labor $=(12/20)/(1/6)=$ $$3\mathrm{.}6$ per part

b$.$ Resource $2$ has a processing time of $3$ minutes per unit.

c$.$ Resource $3$ has a processing time of $5$ minutes per unit.

All three resources are staffed by one worker.