bicycle.rb

require_relative "wheel"
require_relative "gear"

class Bicycle
  attr_reader :wheel, :gear

  def initialize(wheel = Wheel.new(:racing), gear = Gear.new(:high))
    @wheel = wheel
    @gear = gear
  end

  def speed
    wheel.diameter * gear.ratio
  end
end

gear.rb

class Gear
  attr_reader :speed

  RATIOS = {
    high: 10,
    mid: 6,
    low: 2
  }

  def initialize(speed = :mid)
    @speed = speed
  end

  def ratio
    RATIOS[speed.to_sym]
  end
end

test.rb

require_relative "bicycle"

def runtest(expression)
  puts expression ? "Passed" : "Failed"
  # puts  if expression
  #         "Passed"
  #       else
  #         "Failed"
  #       end
end

def test_default
  puts "A new Bicycle has a default wheel type of 'racing'"
  puts "and a default gear of 'high'"
  b = Bicycle.new
  runtest(b.speed == 300)
end

def test_childrens
  w = Wheel.new(:childrens)

  puts "A child's bicycle in low gear has a speed of 48."
  g = Gear.new(:low)
  b = Bicycle.new(w, g)
  runtest(b.speed == 48)

  puts "A child's bicycle in mid gear has a speed of 144."
  g = Gear.new(:mid)
  b = Bicycle.new(w, g)
  runtest(b.speed == 144)

  puts "A child's bicycle in high gear has a speed of 240."
  g = Gear.new(:high)
  b = Bicycle.new(w, g)
  runtest(b.speed == 240)
end

test_default
test_childrens

wheel.rb

class Wheel
  attr_reader :type

  def initialize(type = :touring)
    @type = type
  end

  def diameter
    lookup[type.to_sym]
  end

  private

  def lookup
    {
      racing: 30,
      mountain: 26,
      touring: 28,
      childrens: 24
    }
  end

end