book_in_stock_5.rb

#!/usr/bin/env ruby

class BookInStock

	attr_reader :isbn, :price

	def initialize(isbn, price)
		@isbn = isbn
		@price = Float(price)
	end
	def price=(new_price)
		@price = new_price
	end

	# ..
end

book = BookInStock.new("isbn1", 33.80)
puts "ISBN = #{book.isbn}"
puts "Price = #{book.price}"
book.price = book.price * 0.75
puts "New price = #{book.price}"