Machine Operation. Suppose that two machines 1 and 2 in a factory are operated independently of each other. Let A be the event that machine 1 will work properly during a given 8-hour period, let B be the event that machine 2 will work properly during the same period, and suppose that Pr(A) = 5/6 and Pr(B) = 10/11. We shall determine the probability that at least one of the machines will work properly during the given period.