help with a problem of probability combinations

my first post:

let's see. I was solving a problem of probability combinations, but I had a little problem in dealing with a question, here is the problem:

"a student must meet a test that consists of 3 questions randomly selected from a list of 100 questions (each question has the same probability of being selected). to approve the review he needs to answer at least two questions correctly. What is the probability that the student approve the test if he only knows the answers to 90 questions on the list?"

I started getting the following:

(i) (100 combination 3) = 161700 ........... number of possible questions in groups of 3

(ii) (90 combination 3) = 117480 ............ number of possible questions in groups of 3 students know

really did not know which is the total probability, if 1 / 100 (for each question) or 1 / (100 combination 3) (for each group of questions). (Worried)

then: the 10 questions that the student does not know how it affects the probability?

very grateful in advance, a response