class Faker::Bank

def checksum(num_string)
  num_array = num_string.split('').map(&:to_i)
  (
    7 * (num_array[0] + num_array[3] + num_array[6]) +
      3 * (num_array[1] + num_array[4] + num_array[7]) +
      9 * (num_array[2] + num_array[5])
  ) % 10
end