class Array
  def add
    a = self
    @r = []
    0.upto(a.first.size - 1) do |i|
      @r << a.map { |e| e[i] }.inject(0) { |sum, n| sum + n.to_i }
    end
    @r
  end
end

# [ [4, 6] ]
[ [1, 2], [3, 4] ].add # => [4, 6]