Java code - How do I calcaulate the number of students (who are presumebly 10) that have the ages 18,19,20

In a classroom of 10 students the ages of the students varies between 18 and 20. Calculate the number of students at the ages 18, 19, and 20.

the question is obvious, yet I seriously have no idea what type of approach I have to begin with to write this java code, I appreciate any help.