I have three errands to take care of in the Administration Building. Let Xi
the time that it takes for the ith errand

(i
1, 2, 3), and let X4
the total time in minutes that I spend walking to and from the building and between each errand.

Suppose the Xis are independent, normally distributed, with the following means and standard deviations: 1
15, 1
4, 2
5, 2
1, 3
8, 3
2, 4
12, 4
3.

I 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 .01?