A mother living in a rural village came to visit her son and grandson in the city, but her son gave her a piece of paper and asked her to wait for him in the street. When will the son be back, and what's on that piece of paper?
Overview：A mother living in a rural village came to visit her son and grandson in the city, but her son gave her a piece of paper and asked her to wait for him in the street. When will the son be back, and what's on that piece of paper?
