Blonde Hair Brown Eyes
In a particular class, each student has blonde hair or brown eyes; 1/4 of students with blonde hair have brown eyes and 1/3 of the students with brown eyes have blonde hair.
What fraction of the class in total have brown eyes?
This is best solved with a Venn diagram. Let the number of students with blonde hair and brown eyes be $n$. So the number of students with blonde hair will be 4$n$ (as 1/4 have both) and the number of students with brown eyes will be 3$n$ (as 1/3 have both).
From this we can determine that 3/6 = 1/2 of the class have brown eyes.