Nuts to this, it makes no sense.
What does "cost their past" mean?
If you mean
group A in favor: 128
group A opposed: 32
group B in favor: 96
group B opposed: 48
Now if all those infavor (128+96), if one is selected, Pr(A)=128/(128+96)
If a student is chosen at random,
Pr(against)=totalagainst/total students
= (32+48)/(128+32+96+48)