Unconditionally, each of the c+g+j
donuts is equally likely to be the last one. Thus, the probability that the
last donut is a chocolate donut is cc+g+j.
(b)
We are interested in the event that the last donut is chocolate and the
last donut that is either glazed or jelly is jelly. The probability that the
last donut is chocolate is cc+g+j.
Since any ordering of glazed and jelly donuts is equally likely, the
probability that the last one is a jelly donut is jg+j.
Thus, the probability of the desired event is cc+g+j∗jg+j.