In an examination, 80% students passed in Physics, 70% in Chemistry while 15% failed in both the
subjects. If 325 students passed in both the subjects.
Find the total number of students who appeared in
the examination.
(a) 500
(b) 400
(d) 600
(c) 300
answer-500
please explain

Let total students be 100x.
Students passed in physics = 80x (A)
Students passed in chemistry = 70x (B)
Students failed in both = 15x
Students passed in any or both subject = 100x - students failed in both subject = 100x-15x = 85x ( AUB)
Students passed in both subjects = 325 (A intersection B)
Now,
A + B - (A intersection B) = AUB
80x + 70x - 325 = 85x
65x = 325
x = 5.
So, total students = 100x = 100*5 = 500.