28. (10 points) I have three errands to take care of in Talley. Let X, = the time that it takes for the ith errand (i = 1, 2,3), and let X, = the total time in minutes that I spend walking to and from the building and between each errand. Suppose the X 's are independent and normally distributed, with the following means and standard deviations: /1 = 15, 01 = 4, #2 = 5, 02 = 1, /3 = 8, 03 = 2, H4 = 12, 04 = 3. 1 plan to leave my office at precisely 10:00 a.m. and wish to post a note on my door that reads, "I will return by t a.m." What time t should I write down if I want the probability of my arriving after t to be 0.05