9. A coffee shop launched a "50% off coffee" promotion, stipulating: the first cup is at the original price, the second cup is at half price, and the third cup only costs 3 yuan. Xiao Zhou drank 3 cups of coffee that day, with an average cost of 19 yuan per cup. What is the original price of one cup of coffee? $(\quad)$ yuan.
【Answer】 36
【Analysis】Let the original price be 2 parts, then the price of the second cup is 1 part, the third cup is 3 yuan, in total $19 \times 3=57$ yuan; the original price is $(57-3) \div 3 \times 2=36$ (yuan);
【Lecture Comparison】Average problem, Short-term Class Lecture 1.