class Array
  def add
    inject([ ]) do |sums, row|
      long, short = [sums, row].sort_by { |a| -a.size }
      long.zip(short).map { |l, r| l.to_i + r.to_i }
    end
  end
end

p [ [1, 2], [3, "4"], [1, 0, 7] ].add # => [5, 6, 7]