Question

in an examination 30% of the students fail in science,25% fail in History and 15% in both. What is the pass percentage

Answer 1

first, make a venn diagram.
15% fail both. So the 30% who fail science must include the 15% who failed history too. So the ones who failed science only are (30%-15%)=15%.
Similarly, the 25% who failed history include the 15% who failed science too. So the ones who failed history only are (25%-15%=10%).

Now you sum up all these %s: those who failed science only + those who failed history only + those who failed both=40%