Ruby on Rails Students Marks & Grades App

I'm very new to RoR.. So Please forgive me if you feel this question very basic I'm creating an app which takes students marks as input and Grade as output. I've Students_controller.rb , student.rb .

I need to put logic like this:

def grade(m) if m>80 g="A" else g="B" end

I would be really happy if you let me know where I should place this code