grade.rb

class Grade
  include Comparable
  attr_reader :percentage

  def initialize(percentage)
    @percentage = percentage
  end

  def better_than?(other)
    self > other
  end

  def worse_than?(other)
    self < other
  end

  def <=>(other)
    percentage <=> other.percentage
  end

  def hash
    percentage.hash
  end

  def to_s
    if percentage < 20
      'poor'
    elsif percentage < 50
      'unsatisfactory'
    elsif percentage < 70
      'satisfactory'
    elsif percentage < 90
      'good'
    else
      'excellent'
    end
  end
end