It's pretty clear from problem description that they are not giving away the chickens in the afternoon. They lowered the price and came back selling. It's just

It's pretty clear from problem description that they are not giving away the chickens in the afternoon. They lowered the price and came back selling. It's just doesn't happen in the real life that people call giving away selling. Also in natural language you would expect significantly different wording if they all made 35$ before lunch and then decide to just give remaining chickens for free. Another thing is usage of the word sell. You would have hard time selling anyone an idea that sell could mean give away in the context. Understanding how natural language works and being able to write reasonable constraints based on this description is an important skill. Assuming the price could be 0 in the afternoon is just an error on solvers side.