Explanation:

135

Let say failed student = x

Then passed student= 180-x

Average marks= total marks of all students/ no. of students

50= total marks of all students/180

total marks of all students= 50*180

eqn for total marks….

total marks of all students=    average of passed student* no of passed student   +   avg of failed student* no of failed students

50*180=  80*(180-x)  +   40*x

450= 720-4x + 2x

2x=270

x=135