unko.rb

require 'stringio'
require 'csv'
str = StringIO.new(IO.read("SIKS*********.csv")).read.gsub(/\r/, '')

parse = CSV.parse(str)
parse.shift

sum = 0.0
a = 0.0
parse.each do |row|
  if ["A+", "A", "B", "C"].member?(row[6])
    sum += row[5].to_f
  end

  if ["A+", "A"].member?(row[6])
    a += row[5].to_f
  end
end

puts "#{a}/#{sum} = #{a/sum * 100.0}"