diff --git a/demo_file.rb b/demo_file.rb
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diff --git a/solar_system.rb b/solar_system.rb
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+
+# An array of hashes. One hash for each one of the planets in our solar system
+planets = [
+  {
+    name: "Earth",
+    radius: 6371.0,
+    mass: 5.97237*10**24,
+    sattelites: 1,
+    distance_from_sun: 149.6*10**6,
+    stellar_day: 0.997, # earth days
+    orbital_period: 1.00001742096 # earth years
+  },
+
+  {
+    name: "Jupiter",
+    radius: 69911,
+    mass: 1.8986*10**27,
+    sattelites: 67,
+    distance_from_sun: 778.3*10**6,
+    stellar_day: 0.41354, # earth days
+    orbital_period: 11.8618 # earth years
+  },
+
+  {
+    name: "Mars",
+    radius: 3389.5,
+    mass: 6.4171*10**23,
+    sattelites: 2,
+    distance_from_sun: 227.9*10**6,
+    stellar_day: 1.025957, # earth days
+    orbital_period: 1.8808 # earth years
+  },
+
+  {
+    name: "Mercury",
+    radius: 2439.7,
+    mass: 3.3011*10**23,
+    sattelites: 0,
+    distance_from_sun: 57.9*10**6,
+    stellar_day: 58.646, # earth days
+    orbital_period: 0.240846 # earth years
+  },
+
+  {
+    name: "Neptune",
+    radius: 24622,
+    mass: 1.0243*10**26,
+    sattelites: 14,
+    distance_from_sun: 4497.1*10**6,
+    stellar_day: 0.6713, # earth days
+    orbital_period: 164.8 # earth years
+  },
+
+  {
+    name: "Saturn",
+    radius: 58232,
+    mass: 5.6836*10**26,
+    sattelites: 62,
+    distance_from_sun: 1427.0*10**6,
+    stellar_day: 0.43958, # earth days
+    orbital_period: 29.4571 # earth years
+  },
+
+  {
+    name: "Uranus",
+    radius: 25362,
+    mass: 8.6810*10**25,
+    sattelites: 25,
+    distance_from_sun: 2871.0*10**6,
+    stellar_day: 0.71833, # earth days
+    orbital_period: 84.0205 # earth years
+  },
+
+  {
+    name: "Venus",
+    radius: 6051.8,
+    mass: 4.8675*10**24,
+    sattelites: 0,
+    distance_from_sun: 108.2*10**6,
+    stellar_day: 243.025, # earth days
+    orbital_period: 0.615198 # earth years
+  }
+]
+
+
+
+
+
+# The class SolarSystem
+class SolarSystem
+  attr_reader :age, :planets
+  def initialize(age)
+    @planets = []
+    @age = age # In Earth years
+  end
+
+  # Method that returns a Planet object corresponding to a specific planet name
+  def planet_object(planet_name)
+    @planets.each do |planet|
+      if planet.name == planet_name
+        return planet
+      end
+    end
+  end
+
+ # Method for adding a Planet instance to the array @planets
+  def add_planet(planet)
+    @planets.push planet
+  end
+
+  # Method for adding an array of Planet objects to the array @planet
+  def add_planets(list_of_planets)
+    @planets += list_of_planets
+  end
+
+  # Method that returns the names of the planets either as a string or in an array
+  def planet_names(format)
+    name_list = []
+    @planets.each do |planet|
+      name_list.push planet.name
+    end
+    if format == "string"
+      return name_list.join(", ")
+    elsif format == "list"
+      return name_list
+    end
+  end
+
+  # Method that calculates and returns the distance between two planets
+  def distance_between_two_planets(planet1, planet2)
+    return "#{sprintf( "%0.2g", (planet_object(planet1).distance_from_sun - planet_object(planet2).distance_from_sun).abs) } km"
+  end
+
+  # Method that calculates and returns the local age of a specific planet
+  def local_age(planet)
+    return "The local age of #{planet} is #{sprintf( "%0.3g", @age/planet_object(planet).orbital_period)} years"
+  end
+
+end
+
+
+# The class Planet
+class Planet
+  attr_accessor :radius, :mass, :sattelites, :stellar_day, :distance_from_sun, :orbital_period
+  attr_reader :name
+  attr_reader :distance_unit, :mass_unit, :day_time_unit, :year_time_unit
+  def initialize(planet_hash)
+    @name = planet_hash[:name]
+    @radius = planet_hash[:radius]
+    @mass = planet_hash[:mass]
+    @sattelites = planet_hash[:sattelites]
+    @distance_from_sun = planet_hash[:distance_from_sun]
+    @stellar_day = planet_hash[:stellar_day]
+    @orbital_period = planet_hash[:orbital_period]
+  end
+end
+
+
+
+
+
+# Method for printing a numbered list of the planets in the class SolarSystem
+def print_numbered_list(planet_name_list)
+  planet_name_list.length.times do |index|
+    puts "#{index+1} -#{planet_name_list[index]}"
+  end
+end
+
+
+# Create an instance of the class SolarSystem
+# Our planetary system was formated 4.54 billion years ago.
+our_solar_system = SolarSystem.new(4.54*10**9)
+
+# Add planets to the object our_solar_system via the hash planets
+planets.each do |planet_hash| # iterating over the array planets of hashes
+  our_solar_system.add_planet(Planet.new(planet_hash))
+end
+
+
+
+# User request of information about planets
+done = false
+until done
+  # Interact with a user and provide information about the planet the usera choose.
+  puts "\nWhat planet would you like to learn more about? "
+  puts "Enter the number of the planet from the list below.\n(Enter 13 to exit)"
+
+  # Print out of list of planets
+  planet_name_list = our_solar_system.planet_names("list")
+  print_numbered_list(planet_name_list)
+  puts "13 -Exit"
+
+  # Input of the users choice
+  print "Enter planet number: "
+  entered_number = gets.chomp
+
+  if entered_number == "13"
+    done = true
+  else
+    planet_index = -1 + entered_number.to_i
+    choosen_planet = planet_name_list[planet_index]
+    the_planet_object = our_solar_system.planet_object(choosen_planet)
+    print "\n#{choosen_planet}\'s radius is #{the_planet_object.radius} km, it has a mass of #{sprintf( "%0.3g",the_planet_object.mass)} kg\nand the distance from the sun is #{sprintf( "%0.3g",the_planet_object.distance_from_sun)} km.\n#{our_solar_system.local_age(choosen_planet)}\n\n\n"
+
+    puts "Would you like to have this information about another planet?"
+    print "(yes/no): "
+    answer = gets.chomp.downcase[0]
+    if answer == "n"
+      done = true
+    end
+  end
+
+end
+
+
+# User request of finding out the distance between two given planets.
+puts "\nWould you like to know the distance between two planets?"
+print "(yes/no): "
+answer = gets.chomp.downcase[0]
+
+if answer == "y"
+  # Find the distance between two planets
+  done = false
+  until done
+
+    puts "\nWhat two planes do you want to know the distance between?"
+    puts "Enter the numbers of the planets from the list below."
+
+    # Print out of list of planets
+    planet_name_list = our_solar_system.planet_names("list")
+    print_numbered_list(planet_name_list)
+
+    # Input of the users choice
+    print "Planet number 1: "
+    planet1 = planet_name_list[-1 + gets.chomp.to_i]
+    print "Planet number 2: "
+    planet2 = planet_name_list[-1 + gets.chomp.to_i]
+
+    puts "\nThe distance between #{planet1} and #{planet2}\nis #{our_solar_system.distance_between_two_planets(planet1, planet2)}"
+
+    puts "\nWould you like to konw the distance between two other planets?"
+    print "(yes/no): "
+    answer = gets.chomp.downcase[0]
+    if answer == "n"
+      done = true
+    end
+
+  end
+
+end
+
+
+puts "\n\n\n"
+puts "This is a test of the metod \"add_planets\" in the SolarSystem class"
+# Test of the method add_planets in the class SolarSystem
+# Create an instance of the class SolarSystem
+# our planetary system was formated 4.54 billion years ago.
+solar_system_test = SolarSystem.new(4.54*10**9)
+
+# Add planets to the object our_solar_system via the hash planets
+planets.each do |planet_hash| # iterating over the array planets of hashes
+  solar_system_test.add_planet(Planet.new(planet_hash))
+end
+puts "The original planets are now added to the object solar_system_test."
+puts "This is a list of their names:"
+planet_name_list = solar_system_test.planet_names("list")
+print_numbered_list(planet_name_list)
+
+list_of_additions = [
+  Planet.new({name: "Laban"}),
+  Planet.new({name: "Sune"})
+]
+solar_system_test.add_planets(list_of_additions)
+puts "\nHere is the list after adding two Planet instances via a list:"
+planet_name_list = solar_system_test.planet_names("list")
+print_numbered_list(planet_name_list)