class Hash
  def on(num)
    case num
    when Numeric
      result = find_all {|k, v| k.include?(num) if k.is_a?(Range) }

      case result.size
      when 0
        self[num]
      when 1
        result[0][1]
      else
        result.map {|r| r.last}
      end
    when Range
      range = num.to_a
      result = find_all {|k, v| k.respond_to?(:each) ? k.find {|i| range.include?(i) } : range.include?(k) }
      result.map {|r| r.last }
    end
  end
end

hash = {
  1 => :a,
  2..4 => :b,
  4..8 => :c,
  [9, 10] => :d
}

hash.on(2) # => :b
hash.on(4) # => [:b, :c]
hash.on(1..5).inspect # => [:a, :b, :c]