In an exam,90 students passed in science and 60 students failed in science and 80 failed in maths and 30 failed in both then find the number of students passed in both subjects.
There is no way to relate how many failed to how many passed. All we know is that
50 failed only math
30 failed only science
So 110 failed in something
Knowing that 90 passed science tells us nothing about the class size or how many passed math.