Assalam-o-alaikum:
Q3. I don't understand how "c" is the correct answer? I think all the answers are correct.
Q6. I got correct answer but it could have been "c" also.
Q8. This seems to be a math question! I almost got it wrong.
Q11. I think the correct answer should be "a". It cannot be "d" because there are two students from two different universities. We cannot assume that one student happened to be visiting. Or they both are in a different university. Everything fits for answer "a".
Jazak Allah Khaira