A shopkeeper received 3 differently sized vests to be delivered to 3 different customers. His helper delivered them randomly to the customers without worrying about their respective sizes. What is the probability that at least one of them received the right sized vest?

Let the order of correct sizes be (1, 2, 3). The vests can be delivered to the customers in the following ways: (1, 2, 3), (1, 3, 2), (2, 1, 3), (2, 3, 1), (3, 1, 2) and (3, 2,1) = 6 ways. The cases (1, 2, 3), (1, 3, 2), (2, 1, 3) and (3, 2, 1) correspond to at least one right sized delivery OR 4 ways. Therefore, the required probability = 4/6 = 2/3