Real numbers p and q are randomly chosen from the interval 0 to
1, inclusive. If r is given by r = 2(p + q), and p, q, r are
rounded to the nearest integers to give P, Q and R, respectively,
determine the probability that R = 2(P + Q). (As an example, if
p=0.5 and q=0.381, then r=1.762, and so P =1, Q=0, and R=2)