Petra has mastered the proofs for six of ten theorems. Two of the theorems will appear as questions on the final exam. What is the probability that
a) neither of the proofs on the examination will be among those Petra mastered?
b) at least one of the mastered proofs will appear on the exam?
I'm confused please help thank you

There are 10 theorems. 6 have been mastered, meaning that 4 are unmastered. Two of them will appear on the exam.

a.) You need to calculate P(1st. One selected isn't mastered) * P(2nd. One selected also isn't mastered, with the presumption that the 1st. One already selected was unmastered as well)

b.) This is the complementary event to a.) above, so it'll be 1 - (answer to part a. above)