A faulty test question

Joseph G.R. Martinez describes how students in a teaching methods course were frustrated with a word problem that appeared on a competency examination that no one had been able to solve:

The multiple choice answers were: 5, 6, 7, 10. Martinez writes, “The problem seemed simple enough, but none of the students” attempted solutions made sense, and none yielded answers to match the printed choices. Interestingly, all of the students assumed that the problem was solvable and that the difficulty lay in their own understanding or mathematical skills. After all, this was an important examination devised by expert educators to test the knowledge of novices. It seemed reasonable to assume that the items in the test were error free.

“The students were shocked but intrigued when their professor asked, ‘Could there be an error in the question? And if so, what might the error be?’ Once given permission to think the unthinkable, almost immediately several students identified the flaw: the number of students with both brown hair and hazel eyes (17) could not exceed the numbers for brown hair (15) and hazel eyes (10) individually. There had to be an error in the figures printed.