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

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.