The professor and his wife insert two brochures of Maui in one envelope, two brochures of Oahu in another envelope and one brochure each of Maui and Oahu in the third envelope.

When the professor's wife makes a random selection, he removes one brochure and notices that it is a Maui brochure. The professor could have done this either if his wife chose the envelope having both brochures from Maui or if there is one brochure of each of the places in that envelope. In the latter case, the other other brochure is not a Maui brochure. The probability of this is (1/3). In the former case, the other brochure is also from Maui. The probability of this is also (1/3).

As there is an equal probability of the envelope being the one with two brochures from Maui or the one with one brochure from both the places. This gives the probability that the other brochure in the envelope is from Maui as 0.5