Solution: Age of Teacher and Students

require 'date'
class Person
  attr_accessor :name, :birth_date
  def initialize(name, b_date)
    @name = name
    @birth_date = Date.parse(b_date)
  end
  
  def age
    if @birth_date
      tmp_year = Date.today.year - @birth_date.year
      tmp_month = Date.today.month - @birth_date.month
      tmp_day  = Date.today.day - @birth_date.day
      return tmp_year - 1 if  (tmp_month < 0) || (tmp_month == 0  && tmp_day < 0)
      return tmp_year
    else
      nil
    end
  end
  
end
class Teacher < Person
  
end
class Student < Person
  attr_accessor :grade
  
  def initialize(name, b_date, grade)
    super(name, b_date)
    @grade = grade
  end
  
end
def teacher_avg_age(teachers)
  return teachers.collect{ |y| y.age }.inject(0.0){ |result, el| result + el } / teachers.length
end
def student_avg_age(students, grade)
  grade_students = []
  students.each do |x| 
    grade_students << x if x.grade == grade
  end
  students_count = grade_students.size
  average_grade_age = grade_students.collect{ |y| y.age }.inject(0.0) { |result, el| result + el } / students_count
  return average_grade_age.round(2)
end