Let x be the total number of students.
Let n(A) denotes the number of students passed in English and n(B) denotes the number of students passed in Maths.
n(A) = (80/100)x
n(B) = (85/100)x
n(A⋂B) = (75/100)x
Total number of students passed = n(A⋃B) = n(A) + n(B) – n(A⋂B)
= (80/100)x + (85/100)x – (75/100)x
Number of students failed = x – (9x/10)
Given 40 students failed in both the subjects.
So x/10 = 40
=> x = 400
Hence option b is the answer.