class Person
  attr_accessor :purchased, :name

  def initialize(name)
    @name = name
  end

  def purchase(obj,amt)
    obj.value = amt
    obj.purchased_by = self
    (@purchased ||= []).push(obj)
  end

  def use(obj)
    obj.used_by(self)
  end
end

class Item
  attr_accessor :value, :purchaser, :name

  def initialize(name)
    @name = name
    @value = 0
  end

  def value=(amt)
    @value = amt
  end

  def purchased_by=(person)
    @purchaser=person
  end

  def used_by(person)
    (@users ||= []).push(person)
  end

  def calculate
    @users.each do |x|
      puts "#{x.name} pay #{@purchaser.name} #{@value / @users.count} for #{self.name}"
    end
  end
end

x=Person.new('X')
john=Person.new('John')
alice=Person.new('Alice')
bob=Person.new('Bob')
x.purchase(car=Item.new('car'),400)
john.purchase(groceries=Item.new('groceries'),100)
[x,john,alice,bob].each {|p| p.use(car)}
[alice,bob].each {|p| p.use(groceries)}
[car,groceries].each { |i| i.calculate }