In an examination,60 candidates passed science or maths.if 15 passed both subjects and 9 more passed maths than science, find the number of people who passed maths and science, probability that a candidate passed exactly one subject.
If x passed science. then x+9 passed math. Now you know that
x + x+9 - 15 = 60
x=33
Then 18 passed only science, and 27 passed only math.
So P(only one subject) = (18+27)/60 = 45/60 = 3/4
That's the general way to solve this. Usually you have to figure out how many passed both (or all three, or whatever). In this case, there's a shorter way.
60 passed something
15 passed both, so
60-15 = 45 passed only one.