range_arithmetic.rb

%w(+ -).each do |symbol|
  Range.class_eval do
    define_method symbol do |value|
      x, y = [first, last].map { |x| x.send(symbol, value) }
      x..y
    end
  end
end

%w(* / **).each do |symbol|
  Range.class_eval do
    define_method symbol do |value|
      map { |x| x.send(symbol, value) }
    end
  end
end