game.rb

class Game
  def initialize
    @cards = (1..21).collect { |i| i.to_s }
    @solutions = @cards.clone
  end

  def play
    begin
      play_turn
    end while @solutions.size > 1

    puts "Your number was: #{@solutions.first}"
  end

  private

  def play_turn
    display_cards
    input = get_user_input
    possible = get_possible_solutions(input)
    remove_imposible_solutions(possible)
    shuffle
  end

  def display_cards
    display_header_row
    display_card_rows
  end

  def display_header_row
    puts "\tColumn 1\tColumn 2\tColumn 3"
  end

  def display_card_rows
    7.times { |i| puts card_row(i) }
  end

  def card_row(row)
    a = @cards[row]
    b = @cards[row + 7]
    c = @cards[row + 14]
    "\t   #{a}     \t   #{b}     \t  #{c}"
  end

  def get_user_input
    print "Please pick a column: "
    gets
  end

  def get_possible_solutions(column)
    possible = []
    offset = (column.to_i - 1) * 7
    7.times do |i|
      possible << @cards[i + offset]
    end
    possible
  end

  def remove_imposible_solutions(possible)
    @solutions = @solutions & possible
  end

  def shuffle
    @cards.shuffle!
  end

end

Game.new.play