In a certain class consisting of 36 students, some boys anf some girls, exactly 1/3 of the boys and exactly 1/4 of the girls walk to school.

This is what pops to mind:

(1/3)(36) = 12 boys walk to school.

(1/4)(36) = 9 girls walk to school.

What is the greatest possible number of students in this class who walk to class?

I decided to adf 9 and 12 to 21 students walk to school.

Of course, my answer is wrong.