How to do the drainage scheme of beer shop?

Beer shops use beer to attract and retain customers. Generally, beer is on sale. The common method is to buy three bottles and get one bottle, and buy five bottles and get two bottles of beer to benefit customers. The waiter's reward for selling beer is to recycle the bottle cap. For each bottle cap of this beer, the manufacturer gives the waiter 0.2 cents, and a waiter sells 400 -500 pieces of beer in the beer shop a month. Waiters can earn about 500 yuan by recycling bottle caps for a month.