https://exercism.org/tracks/ruby/exercises/assembly-line/solutions/serradura

iteration_01.rb

class AssemblyLine
  CARS_PRODUCED_PER_HOUR = 221.0
      
  def self.calculate_cars_per_hour(speed)
    speed * CARS_PRODUCED_PER_HOUR
  end
  
  def self.calculate_success_rate(speed)
    case speed
    when 1..4 then 1
    when 5..8 then 0.9
    when 9    then 0.8
    when 10   then 0.77
    end
  end

  def self.calculate_production_rate_per_hour(speed)
    calculate_cars_per_hour(speed) * calculate_success_rate(speed)
  end
      
  def initialize(speed)
    @speed = speed
  end

  def production_rate_per_hour
    self.class.calculate_production_rate_per_hour(@speed)
  end

  ONE_HOUR_IN_MINUTES = 60

  def working_items_per_minute
    (production_rate_per_hour / ONE_HOUR_IN_MINUTES).to_i
  end
end

iteration_02.rb

class AssemblyLine
  module Calculate
    def self.cars_per_hour(speed)
      speed * 221.0
    end
    
    def self.success_rate(speed)
      case speed
      when 1..4 then 1
      when 5..8 then 0.9
      when 9    then 0.8
      when 10   then 0.77
      end
    end

    def self.production_rate_per_hour(speed)
      cars_per_hour(speed) * success_rate(speed)
    end
  end

  attr_reader :production_rate_per_hour

  def initialize(speed)
    @production_rate_per_hour = Calculate.production_rate_per_hour(speed)
  end

  ONE_HOUR_IN_MINUTES = 60

  def working_items_per_minute
    @working_items_per_minute ||= (production_rate_per_hour / ONE_HOUR_IN_MINUTES).to_i
  end
end

iteration_03.rb

class AssemblyLine
  SuccessRate = Struct.new(:speed, :percentage, keyword_init: true) do
    @options = [
      { speed: 1..4, percentage: 1.00 },
      { speed: 5..8, percentage: 0.90 },
      { speed: 9,    percentage: 0.80 },
      { speed: 10,   percentage: 0.77 }
    ].map! { |config| new(**config).freeze }.freeze

    def self.detect_for(speed)
      @options.find { |rate| rate.speed === speed }
    end
  end

  attr_reader :speed, :success_rate

  def initialize(speed)
    @speed        = speed
    @success_rate = SuccessRate.detect_for(speed)
  end

  CARS_PRODUCED_PER_HOUR = 221.0

  def production_rate_per_hour
    @production_rate_per_hour ||= (CARS_PRODUCED_PER_HOUR * speed) * success_rate.percentage
  end

  ONE_HOUR_IN_MINUTES = 60
  
  def working_items_per_minute
    @working_items_per_minute ||= (production_rate_per_hour / ONE_HOUR_IN_MINUTES).to_i
  end
end

iteration_04.rb

class AssemblyLine
  SuccessRate = Struct.new(:speed, :percentage, keyword_init: true) do
    @options = [
      new(speed: 1..4, percentage: 1.00),
      new(speed: 5..8, percentage: 0.90),
      new(speed: 9,    percentage: 0.80),
      new(speed: 10,   percentage: 0.77)
    ].map!(&:freeze).freeze

    def self.detect_for(speed)
      @options.detect { |rate| rate.speed === speed }
    end
  end

  attr_reader :speed, :success_rate

  def initialize(speed)
    @speed        = speed
    @success_rate = SuccessRate.detect_for(speed)
  end

  CARS_PRODUCED_PER_HOUR = 221.0

  def production_rate_per_hour
    @production_rate_per_hour ||= (CARS_PRODUCED_PER_HOUR * speed) * success_rate.percentage
  end

  ONE_HOUR_IN_MINUTES = 60

  def working_items_per_minute
    @working_items_per_minute ||= (production_rate_per_hour / ONE_HOUR_IN_MINUTES).to_i
  end
end

iteration_05.rb

class AssemblyLine
  SuccessRate = Struct.new(:speed, :percentage, keyword_init: true) do
    private_class_method :new
    
    @options = [
      new(speed: 1..4, percentage: 1.00),
      new(speed: 5..8, percentage: 0.90),
      new(speed: 9,    percentage: 0.80),
      new(speed: 10,   percentage: 0.77)
    ].map!(&:freeze).freeze

    def self.detect_for(speed)
      @options.detect { |rate| rate.speed === speed }
    end
  end

  attr_reader :speed, :success_rate

  def initialize(speed)
    @speed        = speed
    @success_rate = SuccessRate.detect_for(speed)
  end

  CARS_PRODUCED_PER_HOUR = 221.0

  def production_rate_per_hour
    @production_rate_per_hour ||= (CARS_PRODUCED_PER_HOUR * speed) * success_rate.percentage
  end

  ONE_HOUR_IN_MINUTES = 60

  def working_items_per_minute
    @working_items_per_minute ||= (production_rate_per_hour / ONE_HOUR_IN_MINUTES).to_i
  end
end

The simplest version.

class AssemblyLine      
  def initialize(speed)
    @speed = speed
  end

  def production_rate_per_hour
    succes_rate = 
      case @speed
      when 1..4 then 1.00
      when 5..8 then 0.90
      when 9    then 0.80
      when 10   then 0.77
      end
      
    (221.0 * @speed) * succes_rate
  end

  def working_items_per_minute
    (production_rate_per_hour / 60).to_i
  end
end

And an iteration that adds constants to reduce the magic numbers code smell.

class AssemblyLine
  def initialize(speed)
    @speed = speed
  end

  CARS_PRODUCED_PER_HOUR = 221.0

  def production_rate_per_hour
    succes_rate =
      case @speed
      when 1..4 then 1.00
      when 5..8 then 0.90
      when 9    then 0.80
      when 10   then 0.77
      end

    (CARS_PRODUCED_PER_HOUR * @speed) * succes_rate
  end

  ONE_HOUR_IN_MINUTES = 60

  def working_items_per_minute
    (production_rate_per_hour / ONE_HOUR_IN_MINUTES).to_i
  end
end

class AssemblyLine
  SuccessRate = Struct.new(:speed) do
    def percentage
      case speed
      when 1..4 then 1.00
      when 5..8 then 0.90
      when 9    then 0.80
      when 10   then 0.77
      end
    end

    def coerce(other)
      [other, percentage]
    end
  end

  attr_reader :speed

  def initialize(speed)
    @speed = speed
  end

  CARS_PRODUCED_PER_HOUR = 221.0

  def production_rate_per_hour
    (CARS_PRODUCED_PER_HOUR * speed) * SuccessRate[speed]
  end

  ONE_HOUR_IN_MINUTES = 60

  def working_items_per_minute
    (production_rate_per_hour / ONE_HOUR_IN_MINUTES).to_i
  end
end