In a school test 52% of the students passed in English and 42% passed in Mathematics, while 17% failed in both subjects. The percent of students who passed in both subjects is:

If 17% failed in both, then 83% passed in at least one subject. Counting the two pass lists gives 52 + 42 = 94%, which is 11 percentage points more than 83 because everyone who passed in both has been counted twice. So 11% passed in both. (6% is 100 − 52 − 42, which would be the answer only if nobody could pass in both, and quoting 17% answers the question about failures.)