If to, tp and tm are the optimistic, pessimistic and most likely time estimates of an activity respectively, the expected time t of the activity will be
A father notes that when his teenage daughter uses the telephone, she takes not less than 6 minutes for a call and som? times as much as an hour. Fifteen minutes call are more frequent than calls of any other duration. If these phone calls were an activity in PERT project, then phone calls expected duration will be