def size_of(directories)
      size = 0
      files_in(directories).each do |f|
        size += File.size(f)
      end
      size
    end
def size_of(directories)
      size = 0
      files_in(directories).each do |f|
        size += File.size(f)
      end
      size
    end
Returns the total size of the directories provided