Assume that the cost of roast duck is 10 yuan, the price is/20 yuan, and the gross profit of each roast duck is 10 yuan. Selling 30 roast ducks every day, the gross profit is 300 yuan, and you can earn 1800 yuan one month excluding rent and water and electricity losses. Roast duck is a famous dish in Beijing and Nanjing, which belongs to Beijing cuisine or Jinling cuisine. This dish is characterized by red color, tender meat, mellow taste and fat but not greasy.