Urn 1 contains one white and two black marbles, urn2 contains one black and two white marbles, and urn 3 contains three black and three white marbles. A die is rolled. If 1,2 or 3 show up, urn 1 is selected; if 4 shows up urn 2 is selected; and if 5 or 6 shows up, urn 3 is selected. A marble is then drawn at random from the selected urn. Find the probability that the marble drawn is white?