def k_checksum
      sum = 0
      checksum_values_with_c_checksum.each_with_index do |value, index|
        sum += ((index % 15) + 1) * value
      end
      sum % 47
    end
def k_checksum
      sum = 0
      checksum_values_with_c_checksum.each_with_index do |value, index|
        sum += ((index % 15) + 1) * value
      end
      sum % 47
    end
Calculates the K checksum based on checksum_values_with_c_checksum