2. Denominator should be Number of 1st class passengers - because the conditional probability P(A|B)=
3. Denominator should be Number of 3rd class passengers -same reason as at 2.
4. Test for independent events: A and B are independent if P(A and B) = P(A) * P(B) - apply the test for this case and see if the 2 events in your question pass it or not.