What are some of the toughest exams in the world?

In a lass ,40% of students are girls . Of the total number of boys 50% failed in an exam. What per cent of the boys passed in the exam?

In the half yearly exam only 70% if the students were passed. Out of these (passed in half yearly) only 60% student are passed in annual exam. Out of those who did not pass the half yearly exam, 80% passed in annual exam. What per cent of the students passed the annual exam ?

A person scores 45% of the total marks in the exam and still fails by 40 marks. The passing percentage of the exam is 55%. What is the maximum marks of the exam?