Consider the (simplified) list of activities and predecessors that are involved in building a house, as shown in the file P15_27.xlsx.

a. Draw an AON project network and find the critical path and the minimum number of days needed to build the house. Also, create the associated Gantt chart.

b. Suppose that by hiring additional workers, the duration of each activity can be reduced. The costs per day of reducing the duration of the activities are also given in the file P15_27.xlsx. Find the strategy that minimizes the cost of completing the project within 20 days.