Solution: Calculate Sales Tax

module GSTCalc
  GST_RATE = 10.0
  
  def net_amount
    if @sales_tax_applicable
      (@amount / (100.0 + GST_RATE) * 100.0).round(2)
    else
      return @amount
    end
  end
  
  def gst
    if @sales_tax_applicable
      tax = (@amount - self.net_amount).round(2)
    else
      return 0.0
    end
  end
  
end
class ServiceItem
  include GSTCalc
  attr_accessor :name, :amount, :sales_tax_applicable
  
  def initialize(name, amount)
    self.name = name
    self.amount = amount
    self.sales_tax_applicable = false    # => default no sales tax 
  end
  
end
class Goods
  include GSTCalc
  
  attr_accessor :name, :amount, :sales_tax_applicable
  
  def initialize(name, amount)
    self.name = name
    self.amount = amount
    self.sales_tax_applicable = true     # => default has sales tax
  end
end
foam_roller = Goods.new("Foam Roller", 49.95)
puts "#{foam_roller.name} Net Amount: #{foam_roller.net_amount}, GST: #{foam_roller.gst}"
physio_service = ServiceItem.new("Physio Consultation", 120.0)
puts "#{physio_service.name} Net Amount: #{physio_service.net_amount}, GST: #{physio_service.gst}"
pilates_class = ServiceItem.new("Pilates Classes", 80.0)
pilates_class.sales_tax_applicable = true
puts "#{pilates_class.name} Net Amount: #{pilates_class.net_amount}, GST: #{pilates_class.gst}"