A die is rolled n=4 times. Assume that the die is fair so that each face value on a single roll is equally likely. Let S2 denote the event that a face value of six appears twice in the 4 rolls.
a) Find P[S2]
b) If O1 is the event that the face value of one appears on exactly one of the 4 rolls, find the conditional probability P[S2O1]