heichan.rb

result = Hash.new { |h,k| h[k] = Hash.new(0) }

CSV.read("file_v1.csv")[1..-1].each do |row|
  group_id = row[1]
  black = !row[2].to_i.zero?

  result[group_id][:total] += 1
  result[group_id][:black] += 1 if black
end

CSV.open("file_v2.csv", 'w') do |csv|
  csv << %w[Group Percentage]
  result.each do |group_id, info|
    csv << [group_id, info[:black].fdiv(info[:total])]
  end
end