Assuming Annie is picking persons at random from the the 6 junior boys, 8 junior girls, 16 senior boys, and 12 senior girls, ...
a. Annie has 8 choices from the 42 students: 8/42 = 4/21
b. Annie has 34 choices from the 42 students: 34/42 = 17/21
c. Annie must exclude the 16 senior boys, so has 26 choices from the 42 students: 26/42 = 13/21
(Or you could use the indicated formula: p(girl ∪ junior) = 20/42 + 14/42 - 8/42 = 26/42, as above.)