Mr. Brown's class has 10 boys and 15 girls. What fraction of the students are girls?
There are a total of 10 boys + 15 girls = <<10+15=25>>25 students in Mr. Brown's class.
The fraction of the students that are girls is 15 girls / 25 students = <<15/25=3/5>>3/5. Answer: \boxed{\frac{3}{5}}.
To find the fraction of the students who are girls, we need to divide the number of girls by the total number of students.
Number of girls: 15
Total number of students: 10 (boys) + 15 (girls) = 25
So, the fraction of students who are girls is 15/25.