9. Xiao Zhang participates in a test with ten multiple-choice questions. He gets one point for each correct answer, loses one point for each wrong answer, and gets zero points for not answering. His goal is to score at least 7 points, with 7 points being the passing score. Xiao Zhang is now certain that his answers to the first six questions are correct, and the probability of answering each of the remaining questions correctly is $\frac{1}{2}$. Therefore, Xiao Zhang should attempt $\qquad$ questions to maximize his probability of passing.
The probability of Xiao Zhang passing by doing one more question is $p_{1}=\frac{1}{2}$; the probability of passing by doing two more questions is $p_{2}=\frac{1}{2^{2}}=\frac{1}{4}$; the probability of passing by doing three more questions is $p_{3}=\frac{1}{2^{3}}+C_{3}^{1} \cdot \frac{1}{2^{3}}=\frac{1}{2}$; the probability of passing by doing four more questions is $p_{4}=\frac{1}{2^{4}}+C_{4}^{1} \cdot \frac{1}{2^{4}}=\frac{5}{16}$, so Xiao Zhang should do 7 or 9 questions to maximize the probability of passing.