econimics_test.rb

require 'test/unit'
require './economics'

class EconomicsTest < Test::Unit::TestCase

  def setup
    @world = World.current
  end
  
  def test_population_of_country_with_the_most_people
    assert_not_equal 0, @world.most_populated_country.population
  end

  def test_country_with_the_most_people
    assert_equal 'China', @world.most_populated_country.name
  end

  def test_five_countries_with_highest_inflation
    assert_equal ["Belarus", "Turkey", "Azerbaijan", "Malawi", "Yemen"], @world.highest_inflation_countries(5).map(&:name)
  end

  def test_five_highest_inflation_rates
    assert_equal [244.0, 94.0, 85.0, 83.3, 71.3], @world.highest_inflation_countries(5).map(&:inflation)
  end

  def test_six_continents
    assert_equal 6, @world.continents.length
  end

  def test_countries_belong_to_continents
    assert_not_nil @world.countries.first.continent
    assert_not_equal 0, @world.continents.first.countries.length
    assert_same @world.continents.first, @world.continents.first.countries.last.continent
  end

  def test_you_can_query_whatever_you_want
    europe = @world.continents.find {|c| c.name == 'Europe'}
    assert_equal 'Germany', europe.countries.max {|a,b| a.population <=> b.population}.name
    assert_equal 'Ukraine', europe.countries.max {|a,b| a.total_area <=> b.total_area}.name
    # ..and so on
  end
end

economics.rb

require 'rexml/document'

class Continent
  attr_reader :name, :countries

  def initialize(element)
    @name       = element.attributes['name']
    @countries  = []
  end
end


class Country
  attr_reader :name, :population, :inflation, :total_area
  attr_reader :continent

  def initialize(world, element)
    @name           = element.attributes['name']
    @population     = element.attributes['population'].to_i
    @inflation      = element.attributes['inflation'].to_f
    @total_area     = element.attributes['total_area'].to_f
    located_within  = element.attributes['continent']
    @continent  = world.continents.find {|c| c.name == located_within }
    @continent.countries.push self
  end
end

class World
  attr_reader :continents, :countries

  def self.current
    @world ||= World.new
  end

  def initialize(cia_file='cia-1996.xml')
    # Yes, it will be slow, but it is done only once. So why sacrifice the Ruby-ish way!
    @continents, @countries = [], []
    doc = REXML::Document.new File.new cia_file
    doc.elements.each('cia/continent') { |e| @continents.push Continent.new(e) }
    doc.elements.each('cia/country') { |e| @countries.push Country.new(self, e) }
  end


  def most_populated_country
    countries.max { |a,b| a.population <=> b.population }
  end
  
  def highest_inflation_countries(top)
    countries.sort_by {|c| -c.inflation }.first top
  end
end

Sorry for including the XML :)
Just wanted to make sure it will "just run" on your side.

Nah! I excluded the XML. Please take the newer Gist.

such a nice and clean code !

=P