Rob D. Store is an accountant who prepares tax returns for customers who submit their income information via the Internet. One of Rob's problems is to figure out when he can promise customers that their tax returns will be ready for their signatures and filing.

Rob (who has studied project management in his MBA program) knows there are four tasks that must be completed for each tax return. He is also aware that the durations of these tasks are uncertain and vary according to known probabilities. The precedence relationships and possible task durations are given in this table:

Task Days Prob

Task (1.2) 2 0.6

3 0.2

4 0.2

Task (1.3) 1 0.1

4 0.9

Task (2.4) 7 0.6

9 0.4

Task (3.4) 10 0.5

2 0.5

a. If Rob uses the classic PERT model and wants to give a customer a due date that has at least a 60 percent chance of occurring, what is the earliest date that Rob can promise the customer's tax return will be completed?

b. In reality, what is the earliest date that Rob should promise the tax return will be completed with at least a 60 percent probability that this will occur?