"Mom's Problem" Little Bean came home from school and found that there were three more members in the family: chicken, duck and goose. Little bean asked, "Mom, what are you going to do when you come back with chickens, ducks and geese?" Mom: "Make soup for my little beans." The little bean saw that these three little guys were so cute that she couldn't bear to cook with them. She said, "Mom, keep them. They will lay eggs for us in the future!" " "Mom:" It's not difficult to raise them, but you have to answer my question. "Little bean jumped three feet high with excitement when she saw her mother agree, and said confidently," It's a deal! ""Mom: "When I bought it from the market, the weight of chickens, ducks and geese was 12 kg, of which chickens and ducks weighed 7 kg and ducks and geese weighed 9 kg. Can you know how many kilograms a duck has? The little bean thought for a moment and said, "The duck weighs 4 kilograms." Mom asked again, "How did you get it?" Beans: "Because chickens and ducks weigh 7kg, ducks and geese weigh 9kg, use 7+9 to calculate the weight of a chicken, two ducks and a goose * * *16kg, and subtract the weight of a chicken, a duck and a goose * * *12kg to get a duck 4kg."