In a paint factory, large and small containers must be filled with paint. Two pumps are available. The fast pump can deliver paint at 12 litres per minute, and the slow pump can deliver paint at 6 litres per minute. Kadu begins to use one of the pumps to fill a large container and the other pump to fill a small container. He starts filling the containers at the same time, and, when the small container is half full, he swaps each container to the other pump. Both containers become full at the same time.