.
The probability of failing exam is the COMPLEMENT to the probability of passing it.
P(failing) = 1 - P(passing) = 1 - 0.8 = 0.2. ANSWER
Solved.
==================
To better memorize, keep in mind this joking statement:
If the probability for a sandwich to fall butter side up is 0.2,
then the probability for the sandwich to fall butter side down
is the COMPLEMENT to it.