Since the student is certain to get at least 4 of the questions correct, he is getting at least a 4 on the exam, then. Thus, it is equivalent to having a 6 (10 questions - 4 guarantees) question test and asking about the probability of getting 1 or more on the exam.

We observe that the chances of him getting any one question correct is 1/5, or 20% and of course, the chance of him getting it wrong would be 4/5, or 80%. Find the probability that he gets all 6 questions wrong first. By independent, we can simply multiple and get . So that chance that he gets at least 1 question right is the complement of the previous probability, or .