There are 4 possible outcomes where H=hard, E=easy: HH, HE, EH, EE.
The first letter is the type of exam, the second letter is the type of last question.
The exam probabilities are H=0.80, E=1-0.80=0.20.
When exam=H, the question probabilities are H=0.90, E=0.10;
When exam=E, the question probabilities are H=0.15, E=0.85.
So the combined probabilities for the 4 outcomes are the products of the probabilities:
These combined probabilities sum to 1, which means we've covered all outcomes.
a) 0.20 (as you probably knew)
c) HH+EH=0.72+0.03=0.75 tells us the probability of the last question being hard, but BEFORE taking the exam. But this is a given (AFTER the exam) so we need to know the relative probability, GIVEN the last question was hard, of EH/(HH+EH)=0.03/0.75=0.04.