Hi, the answer to this question was 9 and I couldn't understand why. Can you explain how to solve this problem?

When taking a 18 question multiple choice test, where each question has 4 possible answers, it would be unusual to get 9 ~ or more questions correct by guessing alone. Note: Of course it would be unusual to get 18 questions correct. You need to enter the smallest possible number of questions to make getting "that many correct" unusual. Give your answer in the box above as a whole number