Question

20% of the girls in class have blonde hair. 16 girls do not have blonde hair. How many total students

Answer 1

The problem is to find how many students have either blond hair or glasses and then divide that by the total number of students; that will find the probability.
Since 7 have blonde hair and 9 have glasses, that would be a total of 16 persons if there were no overlap; that is, no blonde wears glasses and nobody who wears glasses is a blonde.
But there are 4 persons who have both blond hair and wears glasses. That means, of the 7 who are blondes, 3 of them do not wear glasses. And, of the 9 who wear glasses, 5 of them are not blondes.
So we have 3 who are blondes without glasses, 5 who wear glasses who are not blondes, and 4 blonds with glasses, for a total of 12 persons.
The probability is, therefore, 12/26 or 6/13.